I went to Vietnam a couple times. If you hang out downtown in the city, you might get a random Jehovah's Witness or Seventh Day Adventist* try to chat you up. "Oh, we can't do missionary work out in the open, so we just do one-on-one conversations like this". Despite the lack of "Jesus saves, die sinner" signs in Hanoi, you can definitely find Catholic and Protestant churches in Vietnam.
The Western press likes to piss and moan about settler nation missionaries that go, without proper visas mind you, to spread their Western versions of Christianity to the DPRK, only to get deported. So am I allowed to enter a white people country without a visa to stir up trouble and expect no consequences???
I'm the furthest thing from an expert on Myanmar. I get everything I know from Burmese friends. But if you look into the minority people situation, many of them are being heavily proselytised by the worst of the Amerikan type. I don't want the Pat Robertson's the world anywhere near struggling people.
*I'm definitely not saying that JWs and SDAs are anywhere near the worst as Christian sects go.
I think Protestantism is a greater threat than Buddhism in Vietnam because that Protestantism is just a means for the West (and more specifically the US because let's face it the majority of evangelicals are USians) to worm their way (back) into Vietnamese society while Buddhism is just part of traditional Vietnamese society. And as we saw with the self-immolation of Thich Quang Duc, Buddhism has played a progressive role in Vietnamese society before while Christianity in the form of Catholicism has always played a reactionary role in Vietnamese society. Of course, the fact that Buddhism is a part of traditional Vietnamese society doesn't give it a pass and there will be reactionary branches and schools that must be crushed.