If you only want to have conversations with the locals on your trip, just learn a lot of vocabulary and put the words together to form a sentence. You should be understood. Compared to English and Spanish, the Vietnamese grammar is very forgiving in the conversational form. I could be wrong, however. I only had a basic education while I grew up there. I read that the hardest part was the pronunciation.
Young people who live there can help you more. If not, I am sure I can help you some.