Seji, this vomiting is not a problem I have, my Crohn's tends to be more extraintestinal I think - but I can tell you from experience, the only way to relate trigger foods to symptoms is to keep a detailed food diary, and really become familiar with how food is digested in your gut, what sorts of things different foods are high in etc.
It really is an education, all by itself ! I manage my CD almost exclusively by diet, so I have spent more time thinking about
what did what when than I care to remember; some things are as we expect fairly immediate, others can take a day or more, or have a gradual effect that eventually reaches a critical level. The latter, you will find, are usually the ones you relax on, and get away with, so start eating more....
The good news is, once you have kept a food diary for a while, you do it automatically in your head, and sorting out the triggers becomes much easier, even if you are really forgetful like me. And it can pay off big time.