But Vega is at least (i don't believe so) 2 months later. The thing is I have a free sync monitor, do you it is worth the wait from getting the GTX 1080ti now ?
Do you have a gpu now? If you have nothing and don't want to wait you could get something cheap form ebay for $30 or whatever your budget allows. Then when Vega is released you can decide whether Nvidia or AMD is right for you. Since you have freesync I think amd would make sense. If you're already considering spending that much on a top tier you should realize that either will be a good performing gpu you just have to be willing to wait for AMD.