VSync: on or off?

I would say you would want vsync on to cap the frame rate to 90 fps unless oculus is somehow doing this in their software.

I have been keeping Vsync off, and yet my game seems to always run at the recommended fps ( or less in a bad situation : p )

If you turn VSync on, the framerate will be limited to your main monitor refresh rate, so in most cases 60Hz and this is something you don't want. Turn VSync off.

The Rift always has VSync on regardless of what you set it to.

Can confirm. You need Vsync off or it will run your Oculus app at 60fps on a 60hz monitor.