I think you'll have a harder time finding something outside of large cities. I'm not sure what your budget is, but housing, especially that can be rented for foreigners, isnt that cheap in Vietnam.
My experience is limited to Hanoi, where I live for 3-5 months each year. Its pretty easy to find a furnished apartment but the price is cheaper the longer you stay. I don't think you'll have much room for price negotiations with only a 3 month stay. There are several places in Hanoi specifically set up for short term rentals of a few weeks to a few months. But as housing stock exceeds demand I've just gone to a housing agent and found an available apartment and had the agent negotiate a few months rental. I havent had a problem with this as having me rent for a few months is better than the apartment sitting empty and no rent at all.
You can look at the expat site the New Hanoian (which also covers other parts of Vietnam) which has a housing classified section to give you an idea of prices and quality. They also have reviews of several housing agents.