Vomiting is not typically a symptom of IBS, although a few people do vomit on occasion. IBS is not an auto-immune disease; you are thinking of IBD (Inflammatory Bowel Disease), which includes Crohn's.
You need to see a doctor about the random vomiting. It is more commonly a symptom of things like a bad gall bladder or hietal hernia/acid reflux than IBS, so you need to get checked for those things and for any signs of infection or parasites. If nothing there, then look to have a colonoscopy to check for IBDs, since I think it is easier, as you suspect, to get additional auto-immune diseases when you already have one. I think Crohn's disease is also more likely to make you vomit than IBS.
Like I said, vomiting is not unheard of with IBS, but it doesn't seem to be very common (at least a lot of people on here don't complain of it) and it seems to be your main symptom. But if your testing for everything else comes back clear, then you just get to enjoy your IBS that much more. 