Throw in some brats and maybe for one of your salad offerings some sort of grilled chicken salad and I think you'll be fine.
I agree with this, but you could even make the salad kind of make-your-own? You could still do it quite cheaply. Boil up a cheap package of cheese tortellini, maybe grill up a breast or two of chicken if you like (although I would think that would lead to people asking for grilled chicken, so maybe don't do that), and offer both as toppings alongside other salad crudites like cucumber and red pepper, and maybe even a can of rinsed black beans or chick peas.This way vegetarians would have another option besides veggie dogs. Add some shredded cheese, and you have a DELICIOUS salad bar with sources of protein for vegetarians and non-vegetarians alike, and you can do for pretty cheap.
I'm a vegetarian. I agree with the previous poster, I don't like them very much and I prefer Garden Burgers. HOWEVER I don't think it's inconsiderate at all to get veggie dogs, esp. if you're serving hot dogs as the theme of your meal and have a decent salad option, too. A lot of vegetarians who don't like certain products are still grateful that a non-vegetarian has bought an item and created demand for it. I am just grateful when people think of me--I don't expect them to bend over backwards to cater to my preferences, just like it would be inconsiderate to go to a BBQ and expect the host to have picked a certain brand of hot dog.
That having been said, just as you wouldn't be offended if a meat-eater doesn't care for something you've chosen (just like some meat eaters might not prefer bratwurst), don't be offended if the vegetarians opt out of the veggie dogs and choose the salad. You had gone out of your way to give them an option, and that's good of you to have done, but they might prefer a salad, and that shouldn't be offensive. There's nothing more annoying to hear, after I've only eaten one or two Boca burgers, than "Geez, what am I gonna DO with the REST of these!!" (If you don't care for them, it's polite to offer the vegetarian guest the extras to take home, but if they decline, DON'T keep pushing.)