Yeah, that's about right. The Winter Mode button does nothing more than start the VX off in 3rd gear to reduce the amount of torque going to the wheels so they don't spin.
It only activates if you are NOT in Power mode, and if you are stopped, and it shuts off again once you get going. (You would need to hit the button again when starting from the next stop sign) I think it might even be limited to when you are in 4-low range, but that might be wrong. When in 4 low, it makes starts SO much smoother!
But using it will not really do anything for you in the rain. If there is so much rain that you are consistently spinng your tires from a dead stop, then maybe you shoudl reconsider if you need to be on the road in the first place. ;-)