Keep it light and loose. You are sure to hit hot sticky weather on any route in Vietnam. The old traveler rule “Less is more” applies here; bulky luggage is an albatross in Vietnam. Fast and light is best. Loose, long-sleeve shirts and long pants, preferably cotton, are recommended.
Shorts are good for swimming but not great for the backcountry, where mosquitoes are ferocious. Also note that shorts are generally worn by children, not adults (although long shorts are more accepted, especially for young men), and for women only rarely (with sporting events being the exception).
Foreign visitors are somewhat exempt from these conventions, but why not go local where we can? A wide-brimmed hat is essential protection from the sun, and some even carry an umbrella to be used either as a parasol or as cover from sporadic rains.
Sandals are acceptable in most arenas. Affordable laundry service is available everywhere, and thin cotton dries quite quickly — great for a bit of sink-washing instead of carrying around heaps of laundry.