Same reason most racial minorities are Democrats: because they've been brainwashed by Dems that the GOP is racist. Vietnamese-Americans are an exception to this, and I wouldn't be surprised if there are a number of Chinese-Americans, Korean-Americans, or Cambodian-Americans that are Republican.
The only reason, the only reason that Vietnamese-Americans support the Republicans in greater numbers is because they were considered more anti-Communist, same reason the Cuban-Americans support the Republicans. But now, like with most minorities, support for them is eroding among the Vietnamese because they see what party better represents their interests.
The old Vietnamese are still Republican. Obviously their grandchildren are not because the Vietnam War and the Cold War aren't relevant for people who weren't even alive during that era.