English...based on the requirements of becoming naturalized. " Able to read, write, and speak basic English (depending on age)" Since a person would have to be a citizen to vote.
" Where I am from in the United States, if you are an immigrant and don't try to learn English, people get mad at you" Where I am from in Texas, it doesn't matter. That said, if you've been there for a long time it would serve your best interests.
I visited the Thai Embassy here in Saigon and get questions answered from as I say the horses mouth. Don't waste time on friends who aren't Thai Embassy officials they don't know.
I was going to comment on the difficulties of learning Vietnamese. However, at least the letters are recognizeable. When I travel to Thailand, I am lost when it comes to signage in Thai.