Just came back from the 1/19 sailing of the Voyager. You have a few options for dining.
First, between specific hours you can order room service from the dining room menu, or at least most of the items. I can't remember the times, but I do know that lunch stops at 1:30. You have the two informal buffets, on the 11th floor pool deck, you have a spot on the 5th floor prominade that serves snacks (sandwiches, cookies, pastries) 24 hours a day, and don't forget Johnny Rockets. You also have the 24-hour room service option which is a lighter menu. The other option is Portofino's which is the alternative restaurant. They do charge a $20 per person service fee. I do understand that the fee is worth it.
Snacks are served in the lounges at around midnight. They are mostly bite sized dessert goodies. These are taking place of the midnight buffet. Although, Voyager had one deck buffet at midnight and the gala buffet.
You will love this cruise/ship.
Hope this helps.