AMD has 2 options and neither of them is good on paper:
1) Launch Vega 10 ASAP for $650 and trounce 1080, but then NV releases 2070/2080 GP104 refreshes, GP102 and full 3840 CC Titan XP Black. This would steal AMD's thunder in a hurry. Then they would be forced to either drop prices and making the early adopters eats the losses. They can keep the prices and then end up getting outsold like crazy as we've seen with 980Ti vs. Fury X.
2) Wait until NV launches refreshes and GP102/Titan XP Black and then undercut them in every segment. The issue is the perception of inferior brand priced lower, and as we know most consumers will pay huge premiums for similar or slower NV hardware (aka $299 280X vs. $399-449 770 2-4GB, $399 290 vs. $499 780, $549 290X vs. $699 780Ti).
There are really only two ways Vega can disrupt the market. The first is if it flat out beats anything NV has in 2017. The second is if it massively undercuts 1080Ti/2080Ti GP102, where AMD offers Titan XP level of performance for $549 and 1080Ti suddenly looks stupid if NV launches it for $749-799.
I think AMD almost has to launch product first since it seems no matter what they do, NV always has a response.