I also think some Vietnamese parents' obsession with Trump a bit odd. My mom supports Trump but my dad does not. I think we should try to look at different perspectives and in the eyes of another person rather than dismiss them though. People always have a reason for supporting who they support.
From my understanding, these are some of the reasons why they might support him/republicans:
1. He is a businessman. I'm sure you know that many of our parents own businesses (nail salons, restaurants, stores, and such). They probably think he is a president that is good for businesses since he is a businessman. Of course, we know that just because you're a "good" businessman, doesn't mean you can run a country well.
2. They believe he is tough on China. He initiated a trade war with China over unfair trade policies. This is something that both democrats and republicans agree on but no one has acted on it before Trump. Another thing, China has been having land disputes with multiple countries in Asia, one including Vietnam. China and Vietnam have been having a dispute over the Paracel Islands and the general area of the South China Sea. Some Vietnamese fishermen boats have attacked and fishermen assaulted at sea by the Chinese military. The younger generation (especially the overseas younger gen) doesn't pay attention much to this issue but the older generation does. They believe that Trump is helping keep Vietnam's and other countries' sovereignty by having patrols in the South China Sea. The one thing is though, the patrols did not start under Trump. The USA has been having patrols in that area since 2015 under Obama. Trump did increase the frequency at which it happens though.
3. Patriotism. I am no patriot, but even I can see that the Trump supporters are significantly more patriotic than Democrats. One of Trump's campaign slogans was "America First". While some Democrats have burned American flags, Trump supporters are flying them all over the place. Now, see this from the perspective of the older Vietnamese immigrant generation. Many of them are patriotic and love the USA. The USA is the country where they escaped from communism. It is the country that allowed them to rebuild their lives and experience freedom. Many of them are thankful to the USA. When they see democrats burning American flags and "destroying the country", they obviously get a very bad impression of the democratic party. This isn't a thing special to Vietnamese American immigrants either. Patriotism as a reason for republican and or Trump support is also seen in other immigrant populations who come specifically from communist or post-communist countries such as Cuba, Eastern European countries, and Russia.
4. Religion. A lot of Vietnamese immigrants who came to the USA because of the war were Catholic. Minus my family and extended family, many of the Vietnamese families I know in the USA are Catholic. Many Catholics are adamantly against abortion. Buddhists are also against abortion, though not as extreme as Catholics. That could be a reason enough to never vote for the democratic party. I have heard someone say that they don't even like Trump but they will vote for him solely because of the abortion issue.
I hope this gave you some perspective as to why some older generation Vietnamese may support Trump and the republican party. If your friends are genuinely curious, you could give them these reasons also. I used to think anyone who supported Trump was "lost" and couldn't possibly understand why they would support him. Now, I always try to understand other people's perspectives rather than dismiss them. I don't want to tell you how to deal with your parents but I think you should just live and let live. Life is too short to fight with your parents over politics.