Hi, sorry for not replying sooner, I was away for a few days.
As far as I know, there is no hostel in Plitvice area. In the central zone there are hotels, though. They're not very cheap, but perhaps you could stay there for one night. Hotel Plitvice and Hotel Bellevue are the ones with good location, with Bellevue being somewhat cheaper (approx. 50 euro per night, without food). Keep in mind you won't be as flexible to move around without a car, so staying in cheaper accomodation out of the immediate Plitvice area is not practical.
Zagreb area is gastronomically non-vegetarian, so I'm afraid you won't be able to enjoy much of the genuine local food. I'm not familiar with vegetarian restaurants, so all I can do is give you this link with a list of vege-restaurants... I saw people recommend Vege Shop and I can personally recommend Ivica and Marica for deserts. Both places are in central, easily accessible part of Zagreb (use Googlemaps to look for exact address), and they have webpages so you can check them out. If you eat fish, you might enjoy some Croatian-Adriatic cuisine, but I have to ask around for good fish restaurants in Zagreb (I'm a meat-eater, sorry )
Croatian wines are generally good; depending which sorts you prefer, you might ask in a restaurant for a house recommendation. For a general overview of wines in Croatia and terminology there's a pretty good Wikipedia entry.
I'm sorry for not being more helpful this time. I'll be away on a trip after August 26, so you can "catch" me until then.