Find volunteer to help us practice visa interview

While you may want an American to more closely simulate the situation, it might actually be better to find Vietnamese people who have successfully gotten through the student visa interview as they will know what kinds of questions are asked. 

The most important thing to know is that the visa interview process is really a lie detector test without the machine.  The interviewer is looking to eliminate applicants who may be seeking a student visa for other purposes like illegal employment or to look for a spouse.  Answer any question with the absolute truth and don't make things up.  If you don't know the answer, say so.  The interviewer is trained to sniff out lies.