I haven't actually. Vomit doesn't come my way very often and if I'm the one vomiting I'm usually less interested in spell work than I am in getting better. I've also not had cause to go scooping up other people's vomit for spell work.
Icky though your question is, it isn't a silly question. Any personal bodily fluid can be used in spellwork and while blood, urine and semen are the preferred substances there is no reason vomit couldn't be used to equally good effect. That said, I prefer to use blood to urine or vomit because the latter are waste products ejected from the body and I see them as having less of a connection with a person than blood which is the life force itself. So from a tag lock perspective vomit is both a rejected substance and hard to get hold of which - probably explains why you don't hear much talk about its use in magic.
You've also got the problem of storing it. It would need to be frozen or at the very least refrigerated. I'm ok with dead mice, dead birds and frozen menstrual blood in the freezer but I do balk a bit at the thought of vomit nestling between the bags of oven chips. I suppose if I had a definite need for it... But then I'm confident I could find a more palatable alternative to improvise with.
If I did have to employ vomit in spell work I would probably use it in a working connected with corrosion or decay due to the acidic nature of the stomach acid. I also might look to use it in connection with the idea of reversion or going against nature as the physical act of vomiting is a negation of the body's natural digestive process. I was chatting about this with my apprentice who added the suggestion that vomiting could be incorporated into hexing with the energies of the retching motion being used to facilitate the working. Taking this idea further you could consume a symbol of the person (write their name on rice paper perhaps) and then vomit it up as a symbol of expelling them from your life and purging yourself of their influence.