My cat (currently age 11) does not have IBD, but he's been a frequent vomiter throughout his life, as well as being prone to diarrhea/constipation if his diet is not just right. I had him on probiotics for a number of years.
I used this one for a couple of years (let me know if this link doesn't work; sometimes I have trouble with Amazon links on this forum):
It did help with his vomiting. HOWEVER, he eventually had a resurgence of vomiting and it stopped working. It's possible that the formulation of this product changed at the time, as the label also changed. I can't be sure as this was a few years ago. We made a dietary change after that, and I haven't used probiotics since.
So, I haven't used this product for a few years now, but at one point it was a huge help for my cat over the span of multiple years. Take that for what you will.
Edit: I don't know what kind of food your cat eats, but when mine was on this probiotic, he was on a mostly dry-food diet. It took a long time for me to get him onto any wet food.
I mixed powdered probiotics with his dry food by placing the food into a tupperware container with a scoop of the powder, covering it, and shaking it up vigorously so all the pieces of kibble were coated.