The single most useful tip I can think of: if a taxi meter's clear plastic enclosure box is broken through or missing (or sitting beside the meter), then the meter has been tampered with, and after a few km will start shooting up very quickly (although the rigged ones usually start at 12k dong, instead of the usual 15k dong). If the meter is tampered-with, you may want to try to agree on a flat fare before setting off. Less of an issue on very short hops.

No matter what commercial injustices you think you may have been dealt, don't argue with anyone. Don't call the police on anyone. There's still quite a bit of "street justice" doled out in Saigon, some of it by the police themselves.

Watch your dong notes. It's easy to be off on the denomination by an order of magnitude.