I read an article the other day, in mens health (Can't help it. Hubby leaves it on the bathroom counter) that might explain this. In the article it said you should not brush your teeth after every meal unless you wait at least 15 mins or you could actually do more harm then good for your teeth.
When you eat, the acid in food softens the enamel of your teeth. So if you brush your teeth right away, you are brushing away the protective enamel of your teeth.
So I'm guessing if you vomit, it would be highly acidic and soften the enamel of your teeth. Not waiting 15 mins before brushing could cause teeth problems down the road.
Just my guess but makes sense to me.