Of course it isn't just Vegan/Vegetarians who do the "No I can't eat at..." routine. But if it is one, the answer is just to ask where they would want to go. Then, if it's not going to work for you, say, "I'm on a tight budget anyway, so I'll just pick up a quick bite..." Sometimes life isn't so complicated as Dilbert makes it seem.
I just hope that people realize that vegans aren't vegans because its supposedly "healthier," its a matter of ethics. Everybody has a different view on what's ethical, and vegans feel it unethical to kill animals for our own personal gain. If someone you know claims to be vegan for health reasons, please tell them they are vegetarian. Personally, I'm a vegetarian who tries not to use any animal byproduct that results in death of the animal. No leather, nothing with gelatin or animal shortening, no black-ink tattoos, etc.
@LSPMartin: There is actually still no scientific evidence for what you just said. There are a few hokus pokus experiments and theories.
But science was never able to recreate the results. And if you can't do that, then your results are wrong.