You just missed Grape Grazing! I've never stayed up in Yarra Valley but Grape Grazing period is the best time to see quite a few wineries in festival mode. I suggest getting in a car and going to about 3 in a day. It's hard to separate them but check out www.grapegrazing.com.au which gives a good overview of many of the wineries in Yarra Valley plus links to accommodation options.
If you haven't already arrived and figured out where you like to eat here are my suggestions.
I agree that Monsoon and La Chaumiere are worth trying for the high end meals. Plus both have a good atmosphere expecially with the bar right next to Monsoon. I thought the sushi was really good.
For more casual lunch option try Cuppa Cappuccino in 3rd Close, Airport Residential. Try Tribes off Beach Rd for outdoors casual (it is part of a hotel by the beach but can't remember the name). Also try Osekan which is local and basic but cheap and right on the beach - but if there is a band at the time you'll be hit with an entry fee. Plus go to the toilet before you go there.
Try Maquis Tante Maria in Labone. They have opened another one in the Shoprite complex but I'm told it is not as good. They serve West African food. And also dinner only and also casual and outdoors off the main rd in Osu defn go to Mama Mia for the wood fired pizza! Champs in Osu serves good burgers (traditional pub set-up) and I hear good big breakfasts on the weekend.