Or will I offend the locals if I do? I hear it's so hot there, and I would really like to be comfortablebut don't want to be insensitive, either. In Bangkok, the tourists can get away with shorts and tank tops. Is Vientiane more conservative? - Betsy - Guest
If you are only walking around the streets to go shopping, sightseeing, and dining in casual restaurants, shorts and t-shirts are perfectly fine (as long as you don't flash too much skin). Locals wear them as well, so you won't have to worry about offending anyone, except when visiting a temple, religious Wat, royal palaces, and government museums and offices.
If you are in town for official business, it is only common sense to dress appropriately for the occasion. Dress neatly and modestly when visiting religious shrines and temples and don't forget to remove your shoes when entering chapels. At some temples, women may be required to wear a Lao skirt over their clothes before entering the place (these are often available for hire in these places).
If you wish to swim in the rivers and waterfalls, it is best to wear at least a pair of short pants and a t-shirt or a sarong over your swim suit especially when bathing in the evening.