I haven't tried this with mushrooms, but I know it works for, like food poisoning and the flu and whatever.

If you know that you are going to vomit, may I respectfully recommend taking two things:  One, an acid reducing pill like Prilosec OTC, Zantac, etc, and you'll need between twenty minutes and two hours to get that into your system.

Then, when you get to the point where you think vomiting is imminent, and if you have the slightest control over this (i.e. - you know you're going to barf, but you can hold it off long enough to get to the bathroom or whatever), take some antacids. 

The Prilosec will reduce the amount of acid your body produces, and the antacids will neutralise whatever is already there.  It's not likely to stop you from vomiting, but it can make the experience slightly less horrific. 

I'm the kind of person who simply can't stand stomach acid in my sinuses and my nose and whatever, and sometimes if you're gonna hurl, you feel like it's coming out of your eyes and your ears, too.  Neutralising the acid as best as possible goes a long way to alleviating that.