PinkHippie - I'll say this from my perspective as a vegan. Sometimes vegans get disappointed in general if someone who they believe understood veganism enough to go vegan, goes back to being non-vegan. I don't think it is _you_. My mom was vegan for a while and still flip flops between vegetarian/pescatarian. I understand though it is her choice and I am proud of the progress that she has made. She still makes me vegan food when I visit
Now about the pizza... I have to say that for one, if I wanted someone to eat a vegan option of something, I'd offer to buy something that is easily vegan. In the US, that is not pizza. I'd never personally offer to pick up pizza for anyone and the idea of having a pepperoni pizza in my car and paying for it... I couldn't do it. I wouldn't mind being at a pizza party and people eating it because that is their choice.
Anyway, I'd say if your parents buy you a non-vegan pizza, that is more than most vegans would do. I'd eat your pizza and not worry about what they think or what you think they think.