Getting an IRS tax number (ITIN) for new Vietnamese spouse

Question - do you plan to move back to the US in the next year or stay in Vietnam?

The reason I ask is if you are moving back in the next year the easiest thing to do is file as single this year and when you arrive here you can simply file an amended return after she gets her SSN.  You will then get and additional refund as married filing jointly with the higher standard deduction.  That's what I did before moving back to the US.

If you are staying in Vietnam it's less of an issue tax-wise as you would get the foreign income tax credit so it wouldn't impact your return as much but then you would have to send the passport or certified copy of the passport.  I don't know if they will accept the Vietnamese "red stamp" certified copy or if you need to do it at the US consulate as I never did that.

...I don't know if they will accept the Vietnamese "red stamp" certified copy or if you need to do it at the US consulate as I never did that. - @SteinNebraska

Some people will disagree with me but I suggest that a two-step process would be the best:

1. Have the passport copy certified at a MOFA (Ministry of Foreign Affairs) office.

2. Then take that certification and her original passport to a notarization appointment at either the US embassy or consulate.

We did something similar once we were married and we wanted to make sure that my wife always had proof of marriage to me. 

Once our copy of the marriage certificate was authenticated by MOFA, the embassy (Hanoi) notarized the authentication by MOFA as being a genuine certification by the government of Vietnam.