Hello danie2020,
Thank you for using the Apple Support Communities. I see you want to know if you can use an unlocked iPhone 11 Pro Max with carriers based in both the United States and Vietnam. Is that right?
I have a few resources that I can share that should help to answer your question: Identify your iPhone model
Year introduced: 2019
Capacity: 64, 256, 512 GB
Colors: Silver, space gray, gold, midnight green
Model number: A2161 (Canada, United States), A2220 (China mainland, Hong Kong, Macao), A2218 (other countries and regions)"
The United States model, A2161, can be used with carriers in Canada, the United States, and the territories listed here: iPhone - View countries with supported LTE networks. While you could use an unlocked device with any supported carriers within the United States and Canada, it is not compatible with carriers in Vietnam.
I hope I've answered your question.
Have a good day.