I would definitely go with the Vaya for light touring over the 520. Its a far more versatile bike that you can also use as a gravel grinder, weekend club rider, Fondo rider, etc.

Last year I did two 1-week tours but keeping gear light on a Trek FX (aluminum frame, carbon fork) that included a lot of gravel roads and about 45-60 mile days.. I did it with 38mm width tires. It was fine and it's sized comfortably, and with a triple front ring and 11-32 cassette in back it has plenty of gearing range, but I could have really used more versatile handlebars. I have a light tour coming up and considered getting a Salsa Vaya, which I liked riding (I also liked the Salsa Journeyman for less $). But in the end I put butterfly handlebars with cushy bar tape on it and for now fixed my hand comfort issues over longer distances. Was able to install it all myself really easily to further save money.

I still have the idea of getting a versatile, steel framed light touring bike, which the Vaya is perfect for, but with wide tires (I have 38's on my Trek and wouldn't mind going wider) I'm not sure frame material is as important, as most of the shock absorption is happening in my tires and getting a new seat post might actually make more sense. I replaced the Bontrager saddle on my FX with a good Fizik and it was a big upgrade in comfort over distances too.