Because I was vegetarian for several years, every now and then, I have a friend who has a date with a vegetarian call and ask me for dining advice. There are a few things you can do, and a couple of things you should absolutely not do, to make the best impression. Some of my best tips:

1. If you are picking the restaurant, check the menu online or call ahead to find out if there will be plenty for your date to eat. Vegetarian selections at restaurants have gotten much better over the years, but, it’s always good to be sure. Indian, Thai and natural foods restaurants are almost always a good bet if you are unsure.
2. Don’t worry about eating meat in front of a vegetarian. I’ve never known any who were sensitive about what other people ate; those that are that hung up probably would not date an omnivore anyway.
3. Do not make “people eating tasty animals” or other veggie jokes. Chances are, your date has already heard them, and any variation you can think of will be as lame as others they’ve heard before.
4. Fish is not vegetarian. Neither is chicken. If you are unsure what foods your date eats, just ask. Some vegetarians eat dairy and eggs, some abstain from all animal-derived foods altogether.
5. Don’t criticize your date’s food choices. This is just plain rude, and will probably not score you a second date. 
6. Don’t expect meat on your plate on nights your date cooks you a meal. An all-veggie dinner won’t kill you, unless you’re secretly a mountain lion or something.
7. If you want to cook for your date, make sure that you don't use ingredients that have meat ingredients in them. Worchestershire sauce, for instance, contains anchovies. Check a vegetarian cooking site for recipes that are tasty and vegetarian friendly.
8. Don’t overthink it. Part of being on any kind of limited diet means occasionally having less than a cornucopia of meal choices when you go out to eat. If your date has been vegetarian for any length of time, he’ll probably have a pretty good degree of aplomb about the occasional limited restaurant meal. As long as you make an effort to see that there’s something tasty on the menu your date can eat most of the time, an occasional meal of salad and bread won’t kill him, either.