Private Dinner for 20-25 at a NAPA Valley Winery?
I highly recommend the Hall Estate in Rutherford - the dinner takes place in the wine caves, and the set-up is STUNNING. I believe our dinner there was catered by Meadowood, but the Hall team is adept at planning these sorts of events and could work with you to find an appropriate caterer. More details here:
It's not a winery, but I would suggest Meadowood. They have a 9 hole executive course as well as a croquet green. You could certainly, combine an activity along with your dinner. They have some private event rooms that can be set up for your dinner. Or you can have your dinner outside next to the croquet green.
There is a lovely room at Black Stallion Winery on the Silverado Trail. I'm not sure of the capacity. I saw it while searching for the supposed new home of Raymond and Co. Cheese and was told there was a hitch as space was needed for the new kitchen.
re: Ian F
I would say the Merryvale / Tra Vigne option is solid. Here are a few other options:
Peju Provence They have a private dining room and can assist with catering.
V. Sattui cellar room - I hate their wine, but some like it.
Greystone - Technically not a winery anymore but in the heart of wine country St. Helena in Napa valley. They are a high end-ish restaurant and are probably the most adept at catering to large groups.
Robert Sinskey - The owners wife (I think) is a chef and they'll do a private banquet for up to 50 people I believe. Will be amazing but VERY expensive.
2555 Main St St, Helena, CA
Tra Vigne Restaurant
1050 Charter Oak, St. Helena, CA 94574