The hardest thing...pronunication!
Don't bother practising beforehand.... It's easier to learn once you're there. Be patient and persevere! The Vietnamese will appreciate your efforts, so don't panic, they'll help you, correct your mistakes and probably tease you a bit...but that's all part of the fun!
Some good news: Vietnam is the only South East Asian country to use a Latin based alphabet, which helps a lot when trying to read or speak Vietnamese. Spelling and grammar is easy enough to follow but, as there are 6 tones and 5 accents, the pronunciation and phonetics can be real tongue twisters.
I suggest you stick to basic vocab and learn it well! Chatting with children is a great way to pick up the language as you'll learn basic words and grammar.