Give a couple of International Reply Coupons to any Vietnamese you expect to write to you. They cost 95 cents at any post office, and can be redeemed abroad for local postage for one international letter. When you return to the US and correspond with the Vietnamese you met, always include a new IRC in your letter. Take packets of US first-class stamps to give to Vietnamese you meet. Be patient, friendly, and polite, even when things go awry.
Baseball caps with American company logos are status symbols.
They also like logos from sports teams, colleges, etc. No shorts on such visits either. Ask permission before taking pictures. Some people in Hanoi think every camera is a Polaroid and will want to swap their permission for your first print. You might take a Polaroid for this purpose.