Both are about the same price. I always have to monitor so if I see a 1080 for under 400, get that?
4790k OK with either Vega 64 or 1080?
Yep, CPU shouldn't effect the GPU. And if there's a 1080 for under $400 and that's cheaper than the Vega 64 then go for that. But considering your current monitor is FreeSync, I'd get the Vega 64 unless there's a good discount on the 1080.