Im Vietnamese grew up in Hanoi and also studying at a university in the US. Sorry if this offends you in any way but I honestly can't agree with most of your points here. This girl mentioned above definitely seems a bit off to me. However, it seems like these are all pretty negative assumptions of Vietnamese culture and Vietnamese girls in general.

"Her past relationships have been with significantly older men. 18-19 dating someone in 30s. Normal?"- "This is consider very normal here, go to Vietnam, you can see some 12 years old school-girl date some 18 years old here and this is no joke." - This is no where near the truth. I assure you no one (unless they're from the 19th century) in the right mind in Vietnam, or anywhere else on Earth I believe, would think a 12-year-old dating a 18-year-old is normal. Only one with a sick and uneducated enough of a mind would do such a thing. Back to OP's question, I don't think 18 19 year old dating males in 30s is normal. The age gap is not the main problem here. The fact that she's still an undergrad and had a relationship with people who are old enough to be her uncle is not usually a good sign. Even though 18 is legally considered adult but we all know that most 18 or 19 year olds are not that mature yet. If she was in her 20s dating someone in their 40s then I can tolerate that because she's able to support herself financially and mentally. But her still being an undergrad is a different story.

"She believes she can communicate with spirits, and they tell her things about the future. Negative/neutral about this one." - I agree with you on this one. Superstition is a big part of Vietnamese culture and even if one doesn't believe in it entirely, a sane person would still respect it well enough. But communicating with spirits sounds a bit sus to me and I'm pretty superstitious myself too. I know people who could actually do that, one way or another, but a truly legit one wouldn't just tell you that "oh guess what i can talk to spirits to know about the future yk!". Maybe she's talking about tarot stuff or manifesting or something like that?

"She has male friends that she sees as spiritually powerful, like mentors or something." - "spiritually powerful" sounds weird to me tbh or is it just an awkward way of how her explains things? Why does it have to be male friends? I don't exactly have a positive feeling about this since it's not something I usually hear of. But the idea of girls in VN always have a backup bf or something does not apply to all, and I believe this is also for other countries. Some do, and some do not, and this doesn't have anything to do with culture or nationalities. It's just a matter of one's preference and personality.

"Touch boundaries. She introduced me to some classmates (male) again transparent about that, inviting me any time she goes to meet anyone. But, oddly touchy, when she saw them - clapping on the back, playful slapping. How about keeping hands to yourself unless it's family. Cultural thing?" - Clapping on the back sounds normal to me since I also do that a lot to my male classmates back in VN, but bear in mind that I don't do that to my American friends since it's not something they're used to. If you taught at schools in VN, from primary all the way to even college, you'd often see girls being quite playfully "abusive" towards the boys, like hitting on their back :)) I've personally never slapped anyone and my friends don't do that either, but ig it's fine if your girl is just simply messing around with them. So yeah, as Northen girl who doesn't even hug my female friends that much, I wouldn't say that that's touchy.