Counter service are usuallly open with the exception of Harbour House. But I suggest you have dinner before the party so you don't miss a minute of the fun.
There were options, but it was limited and a lot closed down sooner than the party did. Last year (going by my memory so YMMV):
Casey's stayed open through the second parade but was packed and had the screens up as they were under construction
Pecos Bill was open until 8 or 9
Cosmic Ray's with a limited selection (hamburger/cheeseburger/chicken sandwiches line only)
And a bunch of snack carts, but as the night went on it got harder and harder to find stuff besides popcorn...
All Ears has the map from last year so it has what was open so you can plan...
I like getting something beforehand and then snacking throughout the night, but that's my preference. One year we did The Crystal Palace (too cute to see all the kids in their costumes), another time we did The Plaza, a third we did Kona Cafe...
We ate at Cosmic rays in Dec. around 6pm before the MVMCP and plan to do the same when we attend the MNSSHP in Sept. We found it better to eat before the activities started that way we didn't miss anything. When we were done, we got in line to meet the 7 Dwarfs and after that the dance parties started in Cosmic Rays and the menu looked limited.