There are many restaurants and eateries in Port Vila, ranging from up-market places catering to tourists and expats, to more low-key establishments. The approximate cost of lunch would be around the 1000-1500 vatu range, depending on where and what you eat. Some examples of prices:
sandwiches, around 450-600 VT
bacon and eggs, 750 VT
burger with fries or salad, around 1000 VT
main meal, eg steak or seafood, 1200-2000 VT
large, fresh-squeezed juice, around 500VT
The traditional dish which you will most likely be offered once during your stay is a root vegetable cake called lap lap. Essentially this either manioc kasava, sweet potato, taro or yam shaved into the middle of a banana leaf with island cabbage and sometimes a chicken wing on top. This is all wrapped up into a flat package and then cooked in hot stones underground till it all melts together into a cake. The best place to pick up some of this is at the food market in the town centre and should cost you about 100 vatu.
Vanuatu's meat is renowned in the region. At the airports you will see signs reminding you to pack the 25kg of meat permitted to other nearby island nations. The reason the meat's so good is that it's all naturally grown, with no feedlots or other problems of westernised mass production. The result of this is that the steaks are very good indeed.