Flat grapes??? Maybe this is what they look like after they have been turned into wine.

. Maybe some resin company will make miniature bunches of grapes to hang from the vines instead of the P/E ones. However, it is a novel idea to add to the choice of vegetation available for diorama building. If some kind of seed can be found small enough (I seem to remember tiny darkish round seeds mixed into some kind of bird feed) they can be glued together to form a cluster resembling grapes.