I'm painting a buttercream cake this weekend for my daughters 16th bday. She's wanting a galaxy themed cake. If I mix the gel colors with the vodka will the vodka evaporate on buttercream. I've read that it does on fondant but what about buttercream? 

Absolut ly  :)

Oh yes, I do it all the time - works best on buttercream with a chilled cake. If you haven't already, check out CorrieCakes on facebook. She has an amazing step by step galaxy cake photo tutorial in buttercream and I've just been dying for an excuse to try it!