It is a very good idea to be present for the interview at embassies with a high fraud rate like Vietnam. Sometimes hey even ask you to prove that you're not blood related to the beneficiary. Also I've been living here since May waiting for this interview because I wanted to be with her, unfortunately it took way more time than I was hoping but less time than I came prepared for so it is ok.
I went to to the windows with her and I was never told to sit down. They asked her the name of the company I worked for in the US, what I did there and where it was. She mispronounced the name of the company, and said the name of the town I lived in instead of the name of the town where I worked at. She had brought tons of pictures but they didn't look at any.