We have occasionally experienced this situation, and have made accommodations by moving the ceremony under our tented pavilion, creating a ceremony area on the dance floor with a processional aisle, and rows of chairs for the immediate family.  We try to keep the east side of the tent open so that you still have a vineyard view as the backdrop during your ceremony.

Guests would be seated at their tables.  The decision to move the ceremony under the pavilion is typically made at the last minute, as weather conditions often clear up, allowing for an outdoor ceremony, sometimes with just a short delay.