Depends on the game and the engine, but a lot of that optimization time is going to happen regardless for the standard game. A lot of that testing and optimization will help the VR side of things regardless. Hell, that used to be part of my main job over a decade ago when I started in the game industry doing PC hardware testing. I don't think there will be that much more overhead specifically due to VR on top of what they already need to do. Especially if you're going to assume the min specs are the ones the Rift have as "recommended" which will likely be higher than the min spec of a title.
You're overstating how difficult this actually will be.
Weren't you asking about the cost of adding VR support to games versus 3D conversion of movies? What does video content for VR have to do with that?
In any case, simply adding VR support to a game will not be more expensive than full-on 3D conversion of a film. I've worked in both spaces (game development and movie post production), 3D conversion is still pretty laborious and not that cheap.