i personally can't stand stuff that's been cooked at the same time as meat. so what we do is cook things like the onion before the meat goes on. it's always safest to assume that a vegetarian won't want to eat stuff that has been cooked with meat - that way you won't run the risk of having a hungry and pissed of vegetarian seething at your function LOL
in terms of easy stuff to prepare for a vegetarian (and that can be consumed by others!), i chop up corn cobs into about 2ish-cm thick slices, chuck in snow peas, a bit of chilli, garlic & black pepper, and cook them in a frypan (or wok) with a little olive oil. you can do this on the bbq as easily as on the stove.
as a vegetarian, i'm just happy at bbqs if there are a couple of salads provided - i don't need bbq'd food, but it's nice if the host has thought out a couple of vegetarian salads. (some suggestions: noodle salad; rice salad with roast pumpkin, pinenuts & spinach; baked potatoes; coleslaw; the corn/snowpea thing is nice & easy)...
another option would be to have some platters of cheeses, dips, fresh vegies, that kind of thing.
hope that helps some!
kaz x