I think in a buffet the idea of a 'main dish' is not necessary, so long as there is a good range of options and enough plates/forks. This is just a personal taste, but I really dislike pastry-based dishes - cold pastry is almost never very good and there's never enough filling. I also hate fake anything, except prhaps vegan mayonnaise.. ( Am also not at all keen on the current fad to include sweet potatoes in most vegan restaurant meals, but that's another story ....)
Baba ganoush or another garlicky aubergine dip - great with pumpernickel; top with fresh dill.
Cauliflower /broccoli satay. Veg - in neat bite-sized florets- very lightly cooked; I like raw cauliflower but that may be going too far for some. Thick coating sauce of peanut butter, garlic, chilli, lime juice, vegan soy sauce. Decorate dish with fresh coriander and thin slices of lime.
As previous poster said - roast/charred broccoli with vinagrette and either chopped fresh chillis or chopped red pepper. Toasted cashew nuts go excellently with this.
Roasted thin leeks with lemon and black pepper and flaked toasted almonds
Golden oldie:
Three-bean salad in a sharp dressing (vegan mayo with a bit of cider vinegar) chopped spring onions and very finely chopped green pepper. Could add very, very finely chopped celery and gherkins if liked. Very nutritious.
Home made hummus - WITHOUT tahini- with black olive paste swirled through served with sourdough toast or just warmed french bread.
Potato salad with pesto (cold nice really posh new potatoes dressed with vegan mayo mixed with vegan pesto).
I really like buckwheat 'grains' toasted then steamed and mixed with olive-oil fried onions, fried mushrooms and finely-shredded fried kale. Salt and pepper. Best warm but perfectly tasty cool.
Tabbouleh - parsley (lots and lots and lots) and bulgur salad. Plenty of recipes online.
Vine-leaves stuffed with rice and herbs. You can buy these, mostly from Turkey, in tins.
If you have to have pastry, mushroom vol-au-vonts. Best hot but acceptable cold - have never known any left over. (Filling= thick white sauce - make with oilve oil and plant milk - with lots of cooked finely chopped wild mushrooms (dried or frozen) seasoned with thyme and black pepper. Can add a little vegan soy sauce as well if liked. )
If you can face last-minute deep frying, then various Indian snacks - eg mini onion bhajis - are vegan.
Garlic bread. Use olive oil instead of butter and include plenty of parsley.
Hot crispy potato wedges with home-made tomato/red pepper/onion/paprika sauce. I'd add shredded very crisp lettuce and a bowl of vegan mayo as well.
Guacamole - no need to add cream - with plain tortilla chips.
Roast artichoke hearts (from a can). Many recipes online.