If you have to ask a message board whether or not you should put down that you're black, latino/chicano or native american then you probably don't consider yourself apart of that group.
Yes, you are 1/4 black (you "suppose" lol) but if you've never checked one of those boxes until your medical school app, have no associations or affiliations with those communities nor live your life as that sort of minority then trust that the schools will realize you are doing it for some misguided reason.
It isn't really about a minimum percentage. There are people that are a quarter whatever and were raised to identify with that part of their heritage and consider themselves multiracial members of that community. It does not sound like this is you if you have never identified as black.
You can write it down, and should if you normally write that you
are of black descent, you are what you are, but it's disingenuous and disconcerting if you have never identified as black until now. If you are asked about your ties to the community or your mixed identity during interviews and have nothing to say since you don't even consider yourself mixed, you will come off as naive at best or insincere at worst.
For what it is worth, Vietnamese could be considered URM depending on the university. It is up to them to decide.
Identity can be ever evolving. Maybe you will feel differently about your heritage in the future.