Grape leaves have been used as sourdough starter, but if you pay
attention, from sourdough.rec mailing list, if I remember correctly, and
any number of recipe books I've seen, none mention grape leaves.  The
mention I saw of it other than in N.T. said it wasn't a good way to go
about sourdough making.  I have also worked in a bakery and was
responsible for the rye sour, I never heard anything about grape leaves
there either.
Neil
Portland