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.
