Buffets are generally everyone for themselves - putting out a vegetarian (or gluten free, or fat free) option on a buffet table and hoping that only the people who cannot eat the other dishes will take it is a pretty risky way of doing it, for all the reasons mentioned upthread. I think the best way to do it is to either make the veggie version by request, or to have a separate table, separate from the main one, clearly labelled "for vegetarians/gluten free/fat free/nut free only".
I'm an extremely non-picky omnivore, but American ground beef is one of the few things I try to avoid eating for safety reasons, so I might be tempted by the veggie burgers even though normally I'd prefer a good beef burger to a substitute.
I've also been to BBQ buffets where the veggie option was far superior to the meat options. In one case, it was the annual grad student picnic. The budget was tight, so it was the cheapest pre-prepared burgers you could get at No Frills, and the cheapest buns. The person getting the vegetarian option was a bit of a food snob and didn't like veggie substitutes, so the vegetarian option was things like grilled portabellos and red peppers, which were way more pricy that what everyone else was eating.
Things like side salads, bean salads, mac and cheese, veggie pizza etc are even trickier, because to many people these are just plain food that is there to be eaten. Pizza in particular - I know lots of people who just plain like veggie pizza, and more who avoid meat on pizza for other reasons (avoid salt, for example).