It's possible that some people could have genetic reasons that they wouldn't do as well on a veggie diet -- say, some sort of digestive enzyme deficiency or defect -- but blood type really has nothing to do with it. This whole blood type stuff is based on Japanese pseudoscience from the 1920s popularized by Masahiko Nomi in the 70s, then grafted with other bits of psuedoscience afterwards. It's astrology with antigens.
Celiac disease means you can't digest gluten, can't enjoy most bread or seitan, etc, making it harder to be vegetarian. Soy allergy means you react adversely to soy, restricting your options for veggie foods. O+ blood means jack **** unless your blood is going to be mixed with someone else's, through pregnancy or transfusion.