Thanks everyone for the advice. I also decided to call Salsa themselves and also called Shimano and asked them. For my 1x11 configuration, Shimano tech support says to use THREE 2.5mm spacers, two on the drive side, and one on the non-drive side. That's for fitting the 73mm bottom bracket to a 68mm shell. (They recommend one spacer for fitting their MT800 BB to a 73mm shell.). Salsa on the other hand recommends using only two spacers in total, one on either side. Now I think the guy who built the bike used two spacers, but he put them both on the non-drive side!
The Salsa guy said that the original builder may have done it the way he did because the end result, the shifting was better. Shimano said that it's possible that Salsa's directions differ from theirs because Salsa has learned through experience that one spacer on either side just works better on their Vaya frame.
The Shimano rep told me that 1x11 systems can be tricky, and it occurs to me that the cassette the original builder used, (11-46t) with that huge 46t cog may make it even tricker.
So where am I left? I have a much better understanding, thanks to all the advice I've received, but I'm still not sure what configuration I'm going to use!