Hey, if there's a sign saying no outside food, fine. Otherwise, tough it out, people. Or start offering vegan options. It's that simple.
At any rate, to address the original post, don't go there thinking about dessert. Go there thinking about being social. I did the same thing at dinner last night. About 20 of us went to Bob's Big Boy (guess why he's so big?) and I went along to be social, but I was stuck with french fries, fruit, and microwaved veggies as my only vegan options (good thing I brought a Clif Bar!). The waitress was very cool, actually, even asking me more about what veganism involved, as if edifying herself for future guests or something. I still managed to have a good time because I wasn't fixated on the food, but hanging out with friends.