I'm not familiar with soy paint, but the problem with putting varnish over paint is:

a) the paint may not be done curing yet (latex takes a month, I don't know how long soy paint takes)

b) the paint and varnish may not be compatible

I once bought a mirror at a garage sale with a pretty faux metal finish. However, the finish was always tacky. The entire year I owned it, my hair would stick if I got too close. I suspect it was a similar top-coat compatibility issue (which explains why I got such a huge mirror for such a cheap price).