Avoid specific foods
Being a vegetarian per se does not worsen ibs, but certain foods that you may be eating more often may aggravate it.
No single diet
works for everyone, but ibs can be worsened by wheat, dairy, beans/legumes & certain fruits & vegetables. But eggs, nuts, seeds, meats & many fruits & vegies are ok. For more specific info see http://ibs.About.Com/od/ibsfood/a/ibsdiet.Htm & http://fodmapsdiet.Com/.