Sideways
is adapted from the novel of the same name by Rex Pickett.
The novel took a lot of inspiration from Pickett's life during the early-mid 1990s. In Pickett's own words:
'Back then, the Santa Ynez Valley was a little known wine region north of Santa Barbara. Maybe 50 wineries now more than 250. Very little pinot noir had been planted in the 90s when I started sojourning up there, first to play golf, then to get familiar with the wines. I loathed Los Angeles and the cruel film industry that had brought me so much misery. So I would throw my golf clubs in the car and take off. Soon, instead of golfing, I went wine tasting. I hung out at the Hitching Post restaurant and befriended local winemakers. My fascination deepened. I discovered small, ramshackle tasting rooms in this sylvan paradise a mere two hours from the horrors of LA, and I thought: This is heaven.
On one trip, I brought along my friend Roy Gittens, an electrician on my failed second film. We went wine tasting. I made him laugh. Tasting room after tasting room. More wine. And some golf. And ostrich and pinot at the Hitching Post. At some point during that trip, he suggested I write a screenplay about guys who go wine-tasting. Galvanised, I wrote a screenplay called
Two Guys on Wine
. I didnt like it. There was something about it that didnt work.'
Two Guys on Wine
eventually became the novel
Sideways
.
The above quote is from this article by Pickett at
https://www.theguardian.com/stage/2016/may/15/sideways-author-i-was-ready-to-shoot-myself-then-i-found-pinot-noir

Reasonable question. (I was surprised by this as well.) Napa is also reachable from L.A. in one day's drive. And a far more prominent (and attractive) wine area.
But I guess the book was set in the Santa Ynez Valley.

Because in the book it takes place there, not Napa.

The Santa Ynez valley is not far from Santa Barbara. Buellton, where they first stayed, is in Santa Barbara County.

Santa Barbara is world famous.
Regardless, they were no where near Napa nor Santa Barbara.
"God made man. Then he rested. And on the 8th day, God created George O'Brien."

The area they filmed the movie in is just 3 hours north of L.A. perfect for a road trip for Miles and Jack. It includes Solvang, Buellton, and Santa Maria. Plus the Pinot Noir up there is outstanding. My wife and I visited in the aftermath of the film, and visited several of the wineries and sites including the Hitching Post restaurant. Look into Santa Barbara county jeep tours if you are going to be in that area.

Buellton and Santa Maria are over 300 miles from Napa Valley. They are not part of Napa Valley at all.
if you are north of ventura , it is napa valley,
No, Napa Valley is north of San Francisco, far from Ventura.

to me its the same place,if you are north of ventura , it is napa valley,
if you look at the filming locations its all in buelton and santa maria which IS NAPA VALLEY. they go to Solvang and all those locations there.
so the film is in napa, santa barbara has vine too though

It's much, much closer to Los Angeles, where the movie and television business is located.

And Miles himself is more of a Santa Barbara guy than a Napa Valley guy, Napa is not his style.

Santa Barbara is known for Chardonnay and Pinot Noir.
Miles is a Pinot fan.