At stake in balloting starting this week is the future of Winegrowers of California, which united the two groups to enhance wine promotion. Vintners and growers vote separately; unless both approve the 2-year-old program, it will die June 30. The program assesses growers and vintners to raise a promotional budget--$20 million so far. But the marriage has not been happy, and the prestigious Napa Valley Vintners Assn. has strongly attacked the joint effort.