Vietnam uses Dong, best rates are from the Chinese gold shops around the edges of the main markets.
Cambodia uses the US$ for big transactions and The Riel for smaller ones. IIRC it was 4000 riel to the $. If you paid with $1 for something costing 2000 riel you got your change in riel. You could pay for a $2 tuk tuk ride with 8000 riel if you wanted to. Supermarkets used a more exact rate as the amounts involved were larger and would mount up.
Make sure your notes are relatively new and have no tears or marks on them or they will be refused. US$ is the most widely accepted foreign money, other currencies may be harder to exchange outside of the main centres and the rates will be less favourable.