You wouldn't catch me doing steep turns with the doors off in an R22 for precisely the same reasons. I suffer vertigo in the R22 when I fly 500ft over the top of a mountain and then it rapidly drops away down to a couple of thousand feet. I know it's coming and steady myself for it. I don't suffer such extreme symptoms as you, quite possibly as a result of being ready for it. I also fly an MTO Sport autogyro. If I look straight down the side in that, I can feel the vertigo coming on. As a result, I avoid doing so for too long. I only get it by looking directly downwards over the side, everywhere else is fine.
Basically, the secret is to ease yourself back into it slowly. You know that going up with friends has not reproduced the effects. So go up flying and fly as they did. Gradually expand the envelope. Through experience, you should be able to identify exactly when the effects are going to come on, and either avoid them, or learn to recover from them. I just tell myself over and over to keep flying the aircraft and it's an illogical fear. It passes pretty quickly for me, but only once I've stopped doing what was causing the problem. In all likelihood, you'll never stop yourself from suffering vertigo whilst performing tight turns with the doors off. But you may be able to learn how long you can tolerate it, and I think you will learn how to recover your capacity far more quickly now you have a better idea of what it was, rather than combining it with fright adrenaline from your first encounter.
I wouldn't be too worried about it, and just get up flying again. Think of it as good training for the scenario where you experience a high stress scenario due to a significant inflight failure? You've now learnt that yes, high stress zaps capacity, but you can still manage that stress and keep flying the aircraft.
Good luck
GS