Try to master key Vietnamese words such as "Bao nhiêu tian"? (How much is this? – pronounced: bow nyu thien) and "Dat quá". (Too expensive! – daht quah).
When you are given an initial price try a few "Oi Gioi Oi’s" (Oh my God! – pronounced: oi zoy oi) and you'll find things will go a lot better. Remember – keep things lighthearted.