Some how I feel pvp tends to contradict the general philosophy of divinity games. That doesn't mean it isn't possible (or won't be endorsed by Larian's team) but it just doesn't feel right given the structure and nature of previous divinity games.
Mod. It will not be in vanilla.
You don't know that. And it's just annoying that people say over and over again that it could/should be done with a mod without reading the arguments....... It's probably NOT possible, not with all the features a proper PvP mode should have (matchmaking mechanism/lobby, datamatch (biggest problem), online systems (ladder, tournaments,....)).......
If a server client and support for persistent worlds based on D:OS gets added at some point, it will definitely be quite possible.
You don't need a persistent world for a PvP mode. What you need is some sort of lobby/matchmaking which shouldn't be THAT big problem because Dragon Commander will offer something like that for multiplyer matches against other people and it's coded in the same engine.