What would be the exchange rate that the place in SD giving you? The going rate at the jewelry stores in VN is ~1,830,000.00 VND per $100.00. USD is highly sought after in VN so the rich people will pay slightly more to keep USD instead of VND because it is more stable. It is also very hard to move large amount of VND when you have a business transaction, move money abroad, bank will not sell you money if you don't have contract, and I also believe the government won't allow transfer of VND outside Vietnam. All make it difficult for businesses, so the businesses, especially the small businesses will need USD, which the easiest place to get is from private jewelers...
If you are meeting up your friends, relatives, then just have them take you to the jewelry store to get the best rate and the experience, also be sure to bring as new notes as possible (but avoid brand spanking new ones). Slightly used almost as new $100 notes are the best for exchange. Brand new they afraid fakes, too beat up they will not accept it. Small notes will get less value in exchange rate.