First, let's consider why vegetarians are vegetarians. Some have dietary restrictions, some find it morally wrong to eat animals, some don't like meat or have an aversion to eating anything with eyes. There's probably plenty of other reasons as well but from my limited knowledge of vegetarianism, this is all I can come up with.
I suppose I could possibly see someone being so horrendously outraged at the killing of other animals that even performing an action that merely resembles such a travesty, ie. eating an animal cracker, would throw off their moral compass. Other than that, I'm not really seeing the problem with a vegetarian eating an animal cracker.
In order to strengthen this argument (or not), I created a poll where some random people on the internet voted. 4 out of 5 of the respondents agreed that vegetarians eat animal crackers, 3 of them witnessing such an event. The 5th person who said no was probably just trying to mess with the statistics. Also, I personally asked my friendly neighborhood vegetarian whether she eats animal crackers and she responded positively.
However, despite all yay's to this question there still stands that one nay. Therefore, it is impossible to currently prove that all those in the set of all vegetarians eat animal crackers.