I'm not an expert in organic chemistry, but I like to think I have a pretty solid understanding of basic molecular theory. First of all, which stereoisomers are you looking at? Their molecular structure will affect the vapor pressure, so this is important. Look at the polarity of propanol and propanoic acid. Are they polar? Nonpolar? This tells you quite a bit about the intermolecular forces, and from this you can predict the energy needed to break the hydrogen bonds and whatnot. I'm not quite sure how to figure out the vapor pressure (Clausius-Clapeyron equation maybe? I think this requires a little more information), but hopefully this should give you a good start.