While your young son can't speak right now, he is already busy learning the meaning of words in whatever language or languages he is hearing. In helping your child acquire both English and Vietnamese, do what is natural to your family in having him hear both languages. He will need to hear both English and Vietnamese in many different circumstances in order to acquire sufficient vocabulary to speak them well. Reading books to him right now in either language is another excellent way to help him acquire vocabulary.
You can certainly speak both languages to your son. If you have just been speaking English or Vietnamese to him, don't abruptly switch to using just the other language. Instead, introduce the new language gradually.
Don't expect your son's knowledge of both English and Vietnamese to be perfectly balanced. One language may predominate at times because he is using it more than the other. And don't be surprised if he inserts single words or even phrases from one language into the other. This is not a sign that he cannot distinguish between the two languages. Later on, he will only do this with people who understand both languages.