Preferably vegan or vegatarian.
Just reading your note, sounds like a largish party, stand up or buffet service, later in the evening to hit midnight, so not dinner menu nor sit-down service.
If you have possible cooking & serving staff during the party, consider hot morsels. To heat & replenish during the party. Examples: filled pasta (kreplach or ravioli) boiled& fried. Latkes taking advantage of winter produce (potato, beet, carrot, parsnip etc.). All may be served with a choice of sauces
If continuous hot food not possible, make in advance Mediterranean dishes to serve warm, room temp or chilled. Examples: olive spreads, roasted eggplant, bean fritters, stuffed peppers, artichoke spread, nuts, dried fruits, pickles.
Beyond or beside vegan, add eggs, cheeses, small fish or seafood, cured meats or pates.
None of these foods ever met a sparkling wine it didn't like. Remember the particular labels of champagne from driest to sweetest (extra-brut or brut naturelle; brut; extra dry; sec; demi-sec and doux) and buy according to taste. For a meal, most people prefer the three at the dryer end; and for dessert the three at the sweeter end.
Loma, you really don't have to worry. Sparkling wine goes with just about any dish you can think of. So, whatever you decide on, the bubbly will be perfect.
This https://food52.com/recipes... is a great appy, either passed or piled on a plate.
Prosciutto wrapped shrimp, charcuterie plate with meat and cheese, make ahead mac and cheese with truffle cheddar (AMAZING) in a muffin pan, or oyster clams rockefeller (an oldy but a goody) sauteed polenta circles with mushroom or tomato sauce....we have a progressive dinner in my neighborhood and we eat appetizers all night! Clam chowder (or your favorite warm soup) served in nice containers is great too.
The OP is looking for preferably vegan or vegetarian offerings.
Thought of a beautiful chopped vegan salad on thinly sliced pumpernickel to pass. I had something similar at a restaurant and it looked like abstract art (it was a restaurant at the MOMA). It had a grated carrot salad flecked w a green herb. You could add a section of avocado, roasted red pepper, Vietnamese pickled radish. Mustard underneath the salad, I think. Vietnamese salad rolls are lovely finger food. Vegetable dumplings w dipping sauce. Tiny veggie burgers w picked red onions are surprisingly charming.
The recipe that goes with this photo is from food52 but I can't find it. The peppers are peeled raw with a vegetable peeler (finicky but worth it) and glazed with a tiny bit of oil on the stove, then glazed with sherry or sherry vinegar. So beautiful and neat to pick up by themselves.