The Salsa sizing guide on their site is pretty accurate, I went with their suggestions
http://salsacycles.com/bikes/vaya_frameset/ for my Vaya, and although it looks slightly too big, it feels great on the road. looking at this, you would be wanting a 54 or 55cm frame.
Salsa is the same with other makes in using the 26" wheel size for their smaller frames, going to 700c is a big change is size, can you get a 54/55cm to try?
Are you set on the Vaya? much as I like mine, it's not the lightest bike around (fine on flats, you need low gearing for hills), and if doing any trail riding, the
Mariachi would probably be a better choice, as long as you won't ever need the option of racks.