The Federal Land Bank Assn. of Fresno has foreclosed on three vineyards in the past two years, its first in more than half a century. Those foreclosures and six that are pending only show part of the plight of central San Joaquin Valley raisin and wine-grape growers. Poor crop prices have led to plummeting land values, said association President Ken Billings. Many people who took notes when they sold vineyards have had to take the property back because purchasers cannot make payments, he said. The association's annual report showed increases in delinquent loans last year to 62, up from 38 in 1983 and 19 in 1982. There also was a decrease in the number of new loans that the land bank approved from 209 in 1982 to 129 in 1983 and 79 last year. The association is a cooperative and part of the national farm credit system. It holds about half of the long-term agriculture real estate loans in Fresno County.