I cannot speak to the HTC Vive, but for the Oculus Rift, the GTX 980 Ti and GTX 1070 are sufficient in my book -- unless you're doing mining, especially ice mining in densely populated rings. Then you'll want a GTX 1080, but you'll still likely drop under 90fps.
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viewtopic.php?f=9&t=5478#p66149I was extremely surprised how good my GTX 980 Ti was at planetary landings. It was shooting the mining laser in ice rings that finally got the FPS really down. So a GTX 1070, with the Rift, does very well. The GTX 980 Ti is also sufficient. The GTX 980, much more the GTX 970, will not.
YMMV with the Vive.