I don't like the idea of tanks, because it screws over some characters. What if you're an Unarmed or Melee character? What are you going to do, run at it and punch the barrel off? The enemy you guys have described would be utterly unbeatable for such a character.
Tanks as part of a cinematic event (a la Liberty Prime massacring Enclave vertibirds) or just lying around ruined would be fine, but not as a player enemy that's immune to everything except Tesla Cannons and Plasma Casters.
I also have a hard time believing that tanks could resist energy weapons; I'm convinced that's another reason they're obsolete in the Fallout universe. Power-armored troopers are actually able to take cover from energy weapons fire and can blow a tank apart with them in seconds. Plasma is , as mentioned, an industrial cutting tool adapted for military purposes; it'll go through armor like butter.
Indeed.
What I think, if there would be a tank in-game, is that you should be able to take it out with more than the combative skills. Like maybe with Science, you can hack its computer or something, shut the thing down or at least partially.
Maybe there could be a way to get the hatch open, jump in there and beat the crew to death if you are melee/unarmed. Maybe with a high strength (Hercules' Bigger Cousin) you just rip the hatch open, haha!
(and if you have high Speech, you can talk the tank from shooting you, hahaha!)
But anyways, I'd just say that the tank would be in cinematics, or they could just stand at the base of NCR with the reason that maybe the fuel supply lines get raided all the time, which has rendered the tank a stationary cannon. Rather that, than it being an enemy that is immune to most of all damage in the game.