I also have the Vive. Make sure you've correctly set up and checked the "floor level" and chaperone box using SteamVR.
I inadvertently knocked a tracking station out of alignment, and found myself sitting on the main spar of the airplane, and on the roof of my trucks in ATS, etc.
A realignment brought everything back to normal again.
IIRC, I had to put the seated position down separately inside the chaperone box. Need to look into the settings again myself too.