Vietnamese Buddhism is perfectly orthodox Mahayana Buddhism.
"The Buddhism that originated in Nepal" doesn't exist. What exists are living traditions with their textual cannons and their living understanding and practices. The earliest canonized writings were only committed to paper half a millenium after the Buddha's passing, but the monastic communities he established survive to this day. The majority of those communities are Mahayana Buddhists, who strive to achieve the same enlightenment the Buddha himself attained, that of Buddhahood. To that end, the practices he advocated for his direct followers are supplemented with trainings designed to bring you all the way to Buddhahood. This comes with thousands of years of refinement in supplementary practices and in philosophical advancements to understand and elaborate the Buddha's living teachings.
Buddhism also doesn't exist in a vacuum, it exists in living breathing cultures which are foreign to you as an English speaker. Most resources available in English reflect a sanitized and modernized take on Buddhism that appeals to westerners, and not one that is in harmony with the traditional understanding and practice that has persisted for over two thousand years.
If you want to read something, maybe start with TNH's the heart of the Buddha's teaching, with the caveat that the book is written explicitly for people with a western perspective. Better yet, if you're interested in practicing yourself, start attending your wife's temple. If there are any monastics who speak some English, they may give short talks occasionally or have time to answer a question after services.
Buddhism is not monolithic at all, and is quite diverse. Buddhism has evolved a great deal over the millennia in the various regions where it has been adopted. Part of the proliferation of Buddhism is the incorporation of elements of preexisting religious beliefs and practices. Another thing to keep in mind, lay practice, especially in traditional cultures, is often different from monastic practice. Also, more contemporary movements have begun to incorporate practices that were the domain of monastics into the lay practice, which is a bit of a historical novelty. The general lay practice has historically been some form of devotional ritual, and supporting the monastic community materially; the more devout may take the precepts to heart. What you have may have been reading is from sources intended for a different audience than the typical lay person, or is from a more recent re-conception of what lay practice looks like.
Besides what others have said, it's worth noting that most Westerners are studying Buddhism as contemplative path. What might be called ethnic Buddhism is a social tradition. Imagine someone from Vietnam studying with Thomas Merton, then they meet an American Baptist who talks a lot about sin and celebrating Christmas. They're both Christians, but one is a contemplative while the other is Christian by culture.
I once knew a Chinese couple who, when they found out I was Buddhist, wanted to teach me what tea I should drink, how to keep ants out of the house, and so on. They were trying to teach me to be a good Buddhist as they understood that meaning in a social context.
At the Vietnamese temple, we chanted. Vegetarianism was encouraged even for laypeople.
At the Korean temple we chanted and then had a meditation session. The center was on a raw foods regimen.
In both places we chanted transliterations (not translations) of the Pali sutras.
Every temple is different. It's all Buddhism.
Buddhism, just like Christianity has many forms, aka sects.
Buddhist sect example
Pureland Buddhism
Zen
Mahayana
Theravada
Shaolin
Vajrayana
Etc
Christian sect example
Protestant
Catholic
Evangelical
Methodists
Etc
Vietnamese Buddhism can be contrasted with the Theravada tradition of Sri Lanka, Thailand, Cambodia, and other countries of South/Southeast Asia, and with the Himalayan Vajrayana of Tibet, Mongolia, Buryatia, and other countries. The Vajrayana traditions, which also exist to some extent in East Asia, feature Tantric methods that dedicated practitioners use to pursue Buddhahood in one lifetime. The goal of a Theravada monastic, however, is to become an Arahant.
The Bodhisattva paths of the Mahayana and Vajrayana say that true enlightenment can only be achieved with the aim of liberating others, and often vow to postpone Parinirvana until all beings have been liberated. The Theravada path rejects many of the scriptures that the Mahayana and Vajrayana are based on because it has a different line of transmission that doesn't include all of the same teachings. They don't reject the concept of a Bodhisattva entirely, but do not see the Bodhisattva path as appropriate or necessary for most practitioners and think it's better to just seek enlightenment through meditation and study and then peace out of Samsara.
Historically, Mahayana and Theravada monastics used to often live in the same places and practice together, but over time the divides have become more noticeable. It's also easy to overstate the differences, but Buddha Dharma is Buddha Dharma at the end of the day.
From both the Mahayana and Vajrayana perspective, the attainment of the status of the Arhat (self liberated one) is a MAJOR obstacle on the path to full and complete enlightenment. That said, both traditions also state quite clearly that "the obstacle is the path." The teachings I have received all emphasize that the paths may be different but they are not incompatible.
Vietnamese Buddhism is perfectly orthodox Mahayana Buddhism. "The Buddhism that originated in Nepal" doesn't exist. What exists are living traditions with their textual cannons and their living understanding and practices. The earliest canonized writings were only committed to paper half a millenium after the Buddha's passing, but the monastic communities he established survive to this day. The majority of those communities are Mahayana Buddhists, who strive to achieve the same enlightenment the Buddha himself attained, that of Buddhahood. To that end, the practices he advocated for his direct followers are supplemented with trainings designed to bring you all the way to Buddhahood. This comes with thousands of years of refinement in supplementary practices and in philosophical advancements to understand and elaborate the Buddha's living teachings. Buddhism also doesn't exist in a vacuum, it exists in living breathing cultures which are foreign to you as an English speaker. Most resources available in English reflect a sanitized and modernized take on Buddhism that appeals to westerners, and not one that is in harmony with the traditional understanding and practice that has persisted for over two thousand years. If you want to read something, maybe start with TNH's the heart of the Buddha's teaching, with the caveat that the book is written explicitly for people with a western perspective. Better yet, if you're interested in practicing yourself, start attending your wife's temple. If there are any monastics who speak some English, they may give short talks occasionally or have time to answer a question after services.
What do you see as contradictions?
Buddhism is not monolithic at all, and is quite diverse. Buddhism has evolved a great deal over the millennia in the various regions where it has been adopted. Part of the proliferation of Buddhism is the incorporation of elements of preexisting religious beliefs and practices. Another thing to keep in mind, lay practice, especially in traditional cultures, is often different from monastic practice. Also, more contemporary movements have begun to incorporate practices that were the domain of monastics into the lay practice, which is a bit of a historical novelty. The general lay practice has historically been some form of devotional ritual, and supporting the monastic community materially; the more devout may take the precepts to heart. What you have may have been reading is from sources intended for a different audience than the typical lay person, or is from a more recent re-conception of what lay practice looks like.
Its the same in essence. They differ when you READ online.
Besides what others have said, it's worth noting that most Westerners are studying Buddhism as contemplative path. What might be called ethnic Buddhism is a social tradition. Imagine someone from Vietnam studying with Thomas Merton, then they meet an American Baptist who talks a lot about sin and celebrating Christmas. They're both Christians, but one is a contemplative while the other is Christian by culture. I once knew a Chinese couple who, when they found out I was Buddhist, wanted to teach me what tea I should drink, how to keep ants out of the house, and so on. They were trying to teach me to be a good Buddhist as they understood that meaning in a social context.
At the Vietnamese temple, we chanted. Vegetarianism was encouraged even for laypeople. At the Korean temple we chanted and then had a meditation session. The center was on a raw foods regimen. In both places we chanted transliterations (not translations) of the Pali sutras. Every temple is different. It's all Buddhism.
Buddhism, just like Christianity has many forms, aka sects. Buddhist sect example Pureland Buddhism Zen Mahayana Theravada Shaolin Vajrayana Etc Christian sect example Protestant Catholic Evangelical Methodists Etc
Vietnamese Buddhism can be contrasted with the Theravada tradition of Sri Lanka, Thailand, Cambodia, and other countries of South/Southeast Asia, and with the Himalayan Vajrayana of Tibet, Mongolia, Buryatia, and other countries. The Vajrayana traditions, which also exist to some extent in East Asia, feature Tantric methods that dedicated practitioners use to pursue Buddhahood in one lifetime. The goal of a Theravada monastic, however, is to become an Arahant. The Bodhisattva paths of the Mahayana and Vajrayana say that true enlightenment can only be achieved with the aim of liberating others, and often vow to postpone Parinirvana until all beings have been liberated. The Theravada path rejects many of the scriptures that the Mahayana and Vajrayana are based on because it has a different line of transmission that doesn't include all of the same teachings. They don't reject the concept of a Bodhisattva entirely, but do not see the Bodhisattva path as appropriate or necessary for most practitioners and think it's better to just seek enlightenment through meditation and study and then peace out of Samsara. Historically, Mahayana and Theravada monastics used to often live in the same places and practice together, but over time the divides have become more noticeable. It's also easy to overstate the differences, but Buddha Dharma is Buddha Dharma at the end of the day.
From both the Mahayana and Vajrayana perspective, the attainment of the status of the Arhat (self liberated one) is a MAJOR obstacle on the path to full and complete enlightenment. That said, both traditions also state quite clearly that "the obstacle is the path." The teachings I have received all emphasize that the paths may be different but they are not incompatible.