They should be just fine. Vines that age probably have a root system that is in the water table - I shut the water off to mine about two weeks ago - Interlaken is a favorite

Thanks, Dave. I also Googled the question and found out that in Europe it is common practise not to water at all.
We had a pretty damp summer also.

A couple of weeks ago it might have helped to water about an inch, but now with the fall rains and stratus clouds, I don't think you'd gain anything. Supplemental irrigation in the Puget sound region is helpful only for the time period between Mid July- late August. That would be about three irrigations of perhaps 1 to 1 1/2 inches/irrigation.
A vine 15 years old should have a deep root system, but if you have gravelly excessively drained soil or a cemented hardpan layer, the vine could still enter moisture stress -- even in the Puget Sound Western WA region.
Vines under three years old are still shallow rooted and sure do need the irrigation to get a good start. Labrusca type grapes are more shallow rooted than vinifera and not as tolerant of moisture stress.
Deliberate moisture stress is practiced by some wine growers with the opinion that this enhances the wine quality. But I suggest that you have to carefully monitor the soil moisture and the vine's response. Grapes can handle some 'stress' but they are not sagebrush.