I've visited Vietnam sparsely throughout my childhood and have done a trip from South to North by myself.
Automatically, if you are a foreigner, you will most likely be exempt by most if not all of these inadequately "rude" behaviors
Not asking to pay
When going out, it is usually the person whom "invites" you is the one who pays. This is not limited to restaurants, it could be hotels, tours, or any other activities that you all do together. Even so, you should always offer to pay, otherwise you will be perceived as greedy. Although, I've noticed that this trend is phasing out with the new generation, they would rather go Dutch.
Giving money (Tipping)
When you tip someone for their services, you should say "here's some money for coffee". If you present someone with money without saying anything, they will think that you are looking down on them. This goes for any type of services: waiters, street vendors, taxi, etc.
Drinking by yourself
Whether you're drinking Rựu Vang (wine) in the north or Bia (beer) in the south, you should never drink alone when you are with company. Usually one person will raise their glass when:
they want to give a toast.
to compliment a remarkable statement that had been said
Then, everybody will cheer and drink together. Every. Single. Time.
Eating before the elders
When there are elders present, you should use your chopsticks and pick the most special item on the table (usually meat) and place it in their bowl and (optionally) say "mời bác/chú/cô/etc" (here you go sir/ma'm, etc). Eating before the elders is a big no no. This applies to anyone older: your parents, boss, or that friend who is 2 months older than you.
That's all I can think of right now. Like I've mentioned, foreigners are not expected to know or follow this rubric. But if you do learn them and other cultural knacks along the way, the local Vietnamese will be very impressed and happy that you know and respect their culture.