I googled “vietnamese roman missal” and got Vol XVIII January 2007 on usccb.org that says:
Purchasing the Vietnamese Roman Missal
Contact information for obtaining a copy of the Roman Missal in Vietnamese has been changed.
Copies of the new Missal may be obtained from Reverend John Tran, Our Lady of the
Assumption Church, Claremont, California (phone: 909-626-3596)
Hopefully that information is still accurate. If it is not I recommend asking the Lovers of the Holy Cross, but they might not be in your area. Hmm, there are some Vietnamese Dominican sisters in Texas who might know, as well as any Vietnamese priest you find. There are a lot of priests in the Diocese of Orange in California who might know as well. Uh, the Vietnamese Redemptorist Church of Long Beach may know as well.