Developing Promontory, as both a vineyard property and a wine brand, might from the outside look like a thrill seeker’s gamble. In the foothills of Mount Veeder, straddling Oakville and Yountville’s borderlands, the vineyard doesn’t fall within the boundaries of any AVA. Two seismic fault lines bisect it. Averaging a 40 percent slope, it’s nearly too precipitous for cultivation. Scarcely 10 percent of the 840-acre territory is plantable.