I used to live in wine country, not Napa Valley but another valley a couple of hours south with 53 registered vineyards.
Limo tours are quite common for large groups---you'll often get a narrative of why exactly they're charging so much for their fermented grapes.
The locals would ride their bikes between the vineyards instead. Think Tour de Wine. People don't get sloshed wine tasting.The liver can easily keep up. (Cycling is extremely popular/common/great... on par with wine drinking.)
If it's just for the wine, it's probably cheaper to have the wine shipped to you. They do that, special order. You can even get the bigger ones in stores (I lived about a mile from Concannon). If it's for the "experience" of visiting a big farm that happens to grow grapes and charge $10 for a couple of sips at a table in a barn, Napa is the equivalent of Disneyland.
PS: While you're there, Jay Schafer of tiny house fame lives in Sebastopol, not too far away.
PPS: If vineyards are of interest in and of themselves, nearly all agricultural states at the proper latitude have them. There are vineyards in Michigan and New Jersey too, albeit not as well-marketed/famous as Napa.