This is a very complicated question. As MacGruber wrote, it highly depends on what you want and expect.
Even my old gtx980ti was "enough" for several years. But now I have a Radeon 6900xt, because I wanted more.
If you take a look at the new VR headsets, they are all trending to a (somewhat silly) high resolution, and the use are loving them. For fully using those high resolutions, you will need every power you can get.
Saying this, if you only want this for VaM, be warned that any GPU you will buy won't be enough, because this current oldish version of VaM simply can't make use of the full power of your hardware for several reasons.This should be much better with the future new version. In addition to this, the hardware prices are...