Here is an interesting article relating to being both a Vegetarian and Jewish. However, the issue is a valid one for all vegetarians. This article came from Israel National News.
Q. Is all vegetarian food kosher?
A. Unfortunately, supposed vegetarian food is sometimes not even vegetarian. Restaurants, pizza shops and fast food outlets which focus on meat (obviously t’refah i.e. unkosher) often purport to offer vegetarian options but there is no guarantee that the ingredients really are vegetarian and it is almost impossible for them to be prepared and served in vegetarian utensils.
To claim these foods are vegetarian is nothing short of a confidence trick.
Even so-called vegetarian restaurants are not necessarily without major problems; for instance, many brands of bread, oil, cheese, margarine, fat, mayonnaise and even chocolates or other sweets contain meat derivatives. So-called health food shops frequently sell non-kosher meat items.
When vegetarians go shopping it is unwise to rely on product labels; innocent-sounding ingredients can be highly suspect, they can contain admixtures of unacceptable items, and products are often processed on the same machinery used for meat-based foods.
If all this means that the term “vegetarian” can be a misnomer for vegetarians, it can also make life difficult for the kosher consumer.
To be kosher and vegetarian one needs to use the official kosher food directories but judiciously recognise and not utilise fleishig (meat) items.
A question often asked is whether a kosher-observant Jew can patronise a reliably vegan restaurant, but since this involves issues such as Sabbath cooking and other forbidden activities on the part of owners, festivals (the same, as relevant), a proper checking of fruit and vegetables for insects, and the halakhic consideration of “bishul akum” (non-Jewish cooking), a rabbi should be consulted directly.