is she an indoor only kitty? thinking there could have been something she got into outside?
this is usually a symptom of an underlying digestive problem, infection, heartworms or even cancer.
Could be CRF(chronic renal failure) or Hyperthyroidism. do you know if the Vet checked all this out?
If she continues to vomit, she will become dehydrated very quickly and this can be very serious....
It could be from so many causes like I've mentioned and the only way to pinpoint it is thru even more extensive testing...yeah I know its expensive.
Given that she is a senior kitty, I would really suspect it could be HyperT. and if the clinic didn't test by giving a full thyroid panel I would suggest you do take her back and ask for this.
sure wishing you and kitty our best, its really hard to give you much more info than what a Vet could tell you :(( keep us update okay.