Well, darling girl, you're in luck since I happen to live very near the Napa Valley, so I know all about the crazy weather here and can definitely help in the wardrobe department!
As a general rule, June is very mild here in the Bay area, with the weather warming up in July and getting nice and toasty by August and September. As for the Napa Valley area, it is usually a bit warmer there than in San Francisco, because it is further inland. The evenings can get cool, however, so I would always keep a light sweater or wrap handy.
As for wine country style, follow the KISS rule (Keep It Simple Sweetie!). I would opt for classic style. For day wear - touring the vineyards, lunch at the local bistro, etc. - a classic sheath dress or sundress paired with comfortable yet stylish sandals is a great choice. As for accessories, less is more. Understated earrings, necklace or bangles, and a great pair of sunglasses to protect your eyes. Finish with a capacious bag for toting your girly necessities: sunscreen, lip gloss, water bottle, etc.
Another option is a comfortable skirt paired with a sleeveless sweater or top. Knee length or just above the knee is best - shorter tends to be a bit uncomfortable for stepping into and out of vehicles, and longer is just frumpy. Again, simple accessories - you don't want to overpower your ensemble. One great accessory is a silk scarf that is large enough to cover your head if you happen to be lucky enough to be cruising in a convertible, or even if it just gets windy in the vineyards. This same scarf can be loosely tied at your neck instead of wearing a necklace, or it can be tied around the handle of your handbag à la Grace Kelly for a distinctive style.
Try to choose separates that can be mixed and matched to extend your wardrobe, that way you won't need a completely different outfit for every single day. This saves on packing space as well as the wardrobe budget! Also, I usually pack light in the accessories department, because these are the things I like to pick up as souvenirs in my travels. A great pair of earrings, an amazing bracelet, a great wrap or scarf. Then, every time I wear this particular item, I am reminded of my wonderful vacation!