I'm not sure either way, and I doubt we'll ever know. My opinion on a few points though:
- If it was a work, I can't see them leaving it for such a length of time before bringing it back into the storyline. Why not do it for Bret's HOF induction instead?
- Bret punching Vince and there being no lawsuit doesn't really hit me as being evidence either way. Who was there when it happened? And what was stopping Bret from claiming Vince laid hands on him first? Also, Vince could have initially decided to take legal action, then thought about it, and realised that this could be the perfect opportunity to take the company to the next level. A lawsuit would drag out, cost him time and money, and maybe he just couldn't be bothered?
- One thing that makes me think it's more likely to be a shoot, is that the repercussions could so easily have gone the other way. If WCW/Bret had played it better when he went over, they could have been the ones to make that step, instead of the birth of 'Mr McMahon'. Vince would have known that, and wouldn't have willfully handed over a golden opportunity like that.