Speaking of lightweight wrappers to Vega, I've been using Vega a bit and I figured out it'd be good to collect the utilities I write to one place, so I made this: https://github.com/metosin/vega-tools
It includes JSON-Schema validation for Vega maps, which has been pretty essential for me.
(It doesn't include anything else :P)
clojure.spec for Vega maps sounds great. In the meanwhile you might want to run the maps through vega-tools.validate/validate
.