The tower will or won't allow you to use it, regardless of the PRL you have in the phone. (If you try to use a Sprint tower that's in the PRL, but that Sprint doesn't allow VM to use, it'll just be ignored, and the phone will search for another tower.) Updating a PRL might give you some new towers - using an updated PRL for a service you're not on (like a Sprint file if you're on VM) will - at best - gain you nothing.