When I go to dinner at other people's houses, I bring something to share that's vegan. That way I know I'll have at least one thing to eat. I also often eat something before going, and have a snack in my pocket.
When you eat a diet out of the mainstream, you learn to take care of your own food needs. At the same time, everyone knows that I'm vegan, and I imagine just thinking about what they can serve me helps them to start paying attention to what they eat, and why.