I didn't watch the film, and won't. Someone was kind enough to upload the video clip here:
A few foreigners here and there on the net praise her Vietnamese, saying it's "legit", "uncanny", spoken "well", "must be dubbed", etc.
With Vietnamese as my 1st language, I couldn't make out what she said except 1 sentence ("Không, tôi đến đây 1 mình"), no matter how many times I watched the clip. Neither can I say if she learnt some specific lines and tried to speak them, or memorised and reproduced a bunch of sounds that might have sounded Vietnamese to the American ear (more like Thai to me). The Vietnamese audience have the same reaction.
Note: I'm commenting, not criticising (if I have problems with anything, it's not with Jennifer Lawrence, but with the people who say she spoke well a language about which they know nothing).
Finally: có ai biết nguyên văn các câu Jennifer Lawrence nói trong film "X-men" là gì không ạ?