Howdy.
I've been running my Z1 for two years now without the vss hooked up. The nav system absolutely does not need it.
I have a 2006 Jetta with the vss on the CAN bus, so it isn't available without an expensive adapter.
At best, the vss signal is used to estimate vehicle position when in a tunnel or downtown area with tall buildings where a sat loss may occur. Even when I am in those areas, my Z1 tracks beautifully.
So to sum up, it has never been hooked up, and my nav works perfectly. Keep in mind, most NAVs are window units which work just fine without the vss as well.
-J