Its easier to just marry her here in the USA and adjust her status that way. She will be your wife, will be able to study still, will be able to travel and you can be together.
You may submit simultaneously with your petition/application (I-130 and I-485) an application for advance parole (I-131) that will give her permission to leave the country. She would have to state the reason in the application for advance parole of why she needs to return to Vietnam. Leaving without this advance parole could be viewed as an abandonment of her adjustment of status application.
Caution: The Advance Parole is not guaranteed admission BACK into the U.S. As with all entrants, she will be subject to inspection at the POE.