The first thing that one needs to understand is that Vietnamese cuisine in not served in courses. Rather one has to order what he or she wants and all the dishes are brought to the table at the same time. The Vietnamese believe in a mixing of flavours and textures, so make sure you order everything the first time around.
Secondly, if you are not sure what is on the menu and have trouble communicating with the authentic Vietnamese waiting staff then you can always resort to making exaggerated gestures. If you want chicken, you may fold your arms into your arm pits and make clucking sounds. If you want beef you can always go, ‘mooooo’ and if its fish you are after, well, good luck with that. The whole point is to try to get your message across to the waiters if they are unable to understand you.
At the same time, some Vietnamese dishes are extremely spicy. To see if a dish is spicy or not stick your tongue out and flap one hand in front of it to suggest the tongue is on fire. If the waiter gives you a puzzled look then do not worry, he or she will get the idea soon enough.
Most Vietnamese restaurants will usually have a manager that can speak some English so do not hesitate to talk to him or her for help when ordering dinner. You can also look around the restaurant and point out what other people might be eating for the waiter to understand what you want to order.