Hi!
Does your VR helmet work outside DCS? I use Oculus Rift my selfe so I do not realy know how the Vive works, but I had a similar problem. I solved mine by disconecting and reconnecting the Rift. In my case the computer didnt recognize my Rift at all.
But if it works outside DCS, try to deactivate VR in the game, exit the game and turn it back on.