Is it a posh party or more of an informal event? Stuff like samosas (with potato/pea filling), spring rolls and onion bhajis often appear as staple vegan buffet food (I'm in a band and play a lot of wedding receptions, so get to pick from a fair few buffets...). Had a nice pasta salad at a gig the other week that looked like red and green peppers, baby sweetcorn, onions, mangetout and some other vegetables (can't remember what now) in a spicy tomato sauce. And depending on the type of event, there might be roasted potato wedges with ground sea salt and freshly-cracked black pepper (posh chips...).
I'm making some cold buffet food for a friend's 30th birthday party this weekend (providing the veggie/vegan options) and I'm doing: waldorf salad using vegan mayonnaise, sausage rolls with Sosmix filling, couscous and roasted vegetables (red onion, butternut squash, peppers, courgettes, mushrooms), pilaf rice with pine nuts, mushrooms, avocado and cherry tomatoes, ciabatta bread sandwiches with avocado/tomato/vegan pesto mayonnaise and hummus/spinach/grated carrot/grated beetroot/beansprout fillings and pasta salad with vegan pesto and butterbeans. And chocolate cake for afters.
Fairly obvious point, but make sure there's something substantial available like wholemeal pasta, rice or bread and that it's not entirely vegetables and salad ingredients. We played at a wedding a couple of months ago that had a rather Atkins-style buffet - mostly plates of tastefully arranged slices of meat, and some roasted vegetables. They were very nice roasted vegetables, but by the end of the gig the (two) meat-eaters in the band were stuffed and the (four) vegetarians and vegans were falling over...
Hope this helps