VSS Wire is for the following...
1) tells the gps how fast the truck is moving so it can calculate how long it is going to take to get where you are going (Estimated Time of Arrival . . . ETA)
2) if your GPS loses Satellite signal for example a tunnel or tall buildings etc... the GPS can try to calculate your position based upon your speed until it can get verification from the satellites. (debatable how well this really works. mine works great on my Eclipse, however my friends pioneer sucks at this)
3) disable you from working with the GPS while driving. or in my case watching movies while driving.
1 possible solution is to do what Jtruesdell did and put a switch on the VSS wire so that you can temporarily disable it.
take a further look around on the forums and on the internet. for some aftermarket stereos there are boxes that you can hook up that will allow you to disable this. Maybe somebody has made something similar.
On my eclipse i just had to tie the "Parking Brake" wire from my HU to ground. Tricks the HU into thinking that the parking brake is on therefore allowing full use of the HU. GPS Still fully functional on my eclipse when i did this.
Hope this helps clear some things up.