For Show!!!!! Well, if they say come back tomorrow, then do that. Don't sit here and speculating what she/you did wrong. But if you want to speculate and can't do a dawn thing about it, let's go!
Just calm down, think of it this way, they can change their mind anytime! You can't think this far, but even if she had her visa in her passport, someone could call the local police and tell lies about her. Guess what Thai? She wouldn't be leaving VN anytime either, until "further investigation on allegation....(fill-in-the-blank)". Keep it under the lit!
Continue: If she were lucky enough to get pass the Vietnam Immigration Custom Inspection @ Tan Son Nhat, say, she were on the plane coming here. Upon her POE, the US CBP Officer can still deny her visa entry! He can send her back to VN still! So, be very careful and focus from now on out, do not rest until she does, in your living! Get her out of the country as soon as she gets her visa first, for everything else, you can do it later once she's here in the US

Good luck and again... calm down.