hi richard i have a grape vine which i have never pruned and had the same situation with lots of green foilage in the second year but masses of fruit in the third,i think it depends on the species of vine, mine is a marechal foch,

I have a Black Hamburg and I prune it back hard every year. and it fruits well. Cut back to 2 or 3 buds on each main stem [I have 2 main stems]. I do mine on Christmas day, gets me out of the house away from the cooking for half an hour. Prune it before the buds start to grow as they will lose a lot of sap if it is actively growing. I then tie it in along half of the greenhouse . the flowers are very small and green and often difficult to spot.
Hope this helps.

SBG is correct about pruning a grape vine before the sap starts rising. This happens very early in the year, so the recommendation is to do it before 1st January