1. Yes, VPP Professional is compatible with multiplayer. It's the same hassle as any regular multiplayer game.
2. Yes, the VPP scripts don't have to be modified for multiplayer. Even in the Enterprise edition, modifying the source code is strongly discouraged, and could cause other issues about which we couldn't provide support.
NetworkBehaviour is required for the scripts that handle the networked elements. For a vehicle, you would create a script like NetworkVehicle derived from NetworkBehaviour. This script would handle all the network elements, and coordinate them with the unmodified VPP components.