I personally haven't been able to stick to a strict vegan lifestyle.
But when I was trying it out, I found it helped not to explain why you do or don't eat something. It's easier just to say, "No thanks" when someone offers you a nonvegan food item, and to simply say, "It sounded tasty" when somebody asks you why you got the veggie sandwich at the cafeteria instead of your usual turkey sandwich.
Of course, when you have coworkers who really like to treat you to cookies and stuff, because they KNOW you love cookies, and you suddenly start turning them down -- they WILL question you and try to change your mind and get you to eat their cookies again! In the cases where you say "No thanks" but they still force food onto you... I'd sneak it into the garbage or pass it on to someone else as soon as they leave. Just because someone gives you food doesn't mean you have to eat it.
I guess one important thing to remember is that food is very social, and by rejecting someone's food, some people feel rejected themselves. You'd think food would be a completely neutral topic, but as you continue your vegan lifestyle, you'll find out that it's extremely tied in with people's emotions, self-worth, and views of the world.
Good luck!
And no, I didn't get the "I'm a vegan, dumbass" feel from your post, so don't worry about it.
I think the problem is more that you don't expect it to be a big deal, but the people in the world who think that chicken is a vegetable (or "bruchetta doesn't count" -- yes, they exist) will have no friggin' clue what you're talking about. I can't tell you the number of times a well-meaning cook I know has gone to length to create me a "vegan" dish, just for me to find out that it's slathered in butter or something.