According to the pilot, he had completed an uneventful flight earlier in the day. On the accident flight, he performed an engine run-up, which included a magneto check with no mechanical anomalies encountered. He taxied to the active runway (runway 13), and noted the winds were light, about 5 knots from the south. The takeoff roll was normal, and they lifted off at 50 mph. After rotation, the airplane started a left bank for no apparent reason. The pilot did not pull back on the control stick; however, he attempted to align the airplane with the runway. He noted that the airplane did not respond to his input and continued in the left bank. The pilot also stated that the airplane had started a slight climb just prior to impacting a vineyard. After the initial contact with the vineyard, the airplane became airborne again and then dropped back into the vineyard. The pilot stated that there were no mechanical problems with the airplane and he believes that the winds may have shifted to more of a south wind at the time of the loss of control event. Reported winds at the time were from 140 degrees at 5 knots.