Most vegans (and Neapolitan pizza lovers) are probably well aware of this fact, but if you are friends with vegan folk, you never, ever have to worry about eating pizza with them if you keep this in mind:
"Most wood-fired places serve a traditional pie called a marinara consisting of tomato, sea salt, oregano, basil (one leaf), olive oil, and garlic. It's almost always vegan, and delicious."
Tonycalzone, thank you. That is some serious, solid advice and a great tip to file away.
On Pizza Cutters
An Important Note: As HerbSperb points out in the comments here, "At least, it's vegan until they run the pizza cutter through it." Good point. Ask if they use a separate pizza-cutter for vegan pies or, if you're at a Neapolitan-style joint, just ask for the pizza uncut and use a knife and fork. Most Neapolitan-style pizzerias will accommodate this request.