Patricia,
I spent over a year in VietNam - I can comment on the environmental conditions. During my year there, I was forced to wear outfits designed by people hung up on greens & browns. Colorful accessories were definitely out! It will be really hot and humid in the Mekong Delta, Saigon, Vung Tau, and along the east coast. Doesn't get cooler until in the highlands such as Pleiku - not less humid, just cooler. Occasional breezes are life-savers. Bugs - flying, crawling, leaping - are a problem. Best to cover up as much as tolerable. 50 to 100% Eau de Deet is essential. May thru October are the wetter months in the south with bit different schedules north & central. Be careful when walking amidst undergrowth plants. Most plants are abrasive, thorny, toxic or bug laden. Or more likely, all of those. It is a tropical rain forest after all. There are times when the countryside can seem serene and lovely. (I never trusted that condition.)
It is probable that the most practical women's garment is the local AoDai. Light weight, good coverage, breezy, quick drying and attractive. Would work well with silk or synthetic fabric long pants. We appreciated the AoDai because it was attractive and difficult to conceal a weapon. No sandals.
Read and follow the CDC guidelines. Get shots and medicines for everything. Even if the chances are small, the consequences of any disease are very serious.
Obviously, my attitudes about SouthEast Asia were formed under circumstances other than "tourist." I felt sorry for the average residents - they just wanted EVERYBODY to leave them alone. Fat Chance. Take appropriate precautions and enjoy your trip.