yup, us vegans are just no fun;) And I think there is a line between declining to go to an office party and getting all worked up and offended when someone just asks what your birthday is. I agree with Lass, if you really are that easily offended, you shouldn’t leave the house. And getting back to the OP who seems to have ditched the conversation, I do believe she was being just a tad hypersensitive and needs to chill out a bit. 
Considering most people nowadays spend more time in the office than at home, I think a little social interaction sprinkled in the workplace is a healthy thing.
Most of these people that are being discussed (the ones who won’t take no for an answer) are annoying yes, but more often than not, their hearts are in the right place. I think instead of flying off the handle and hurting anyones feelings with icy stares or “funny” yet mean spirited practical jokes, you should just let it roll off your back. It’s all about being the bigger person, your karma will thank you for it. The more little things you let irritate the crap outta you, the grumpier of a person you become. 