Well how did you do it when they still ate meat? I'd imagine that's much more difficult. With vegetarians sometimes all they want to do is add some cheese to their dish, or they'll just eat certain things you don't.
I lived with vegetarians who I shared meals with for about a year, and they ate a lot of vegan meals, like tofu and vegetable stir fries, Gardein-based dinners on a sandwich or wrap with a veggie side, veggie burgers, accidentally vegan ramen, and soups or stews that were made from scratch. Mainly I had soy milk and they had dairy milk, though they actually drank hazelnut or hemp milk sometimes. If I made burritos they might add cheese while I had vegan cheese or extra guac. We ate two different kinds of bread sometimes, but shared a lot of basic items. It helped if I made holiday-type dinners, so I could make potato salad with avocado and olive oil instead of mayo, or if I could commandeer what went into the mashed potatoes that I served with the Tofurky.
They were long-term vegetarians though, a woman who had been a vegetarian for over 20 years who also refrained from eating gelatin and bought vegan leather shoes who donated money to PETA, she was an old school ethical vegetarian. Her son had never had meat in his life, but he really really loved dairy. The main thing they ate a lot that I didn't was eggs. She really thought she wouldn't get enough protein if she didn't eat eggs, and it was hard to convince her otherwise.
