Cooking Fresh Grape Leaves -- Can't Chew Them
A friend of mine has a large grape vine in his back yard. I picked a bag of leaves, thinking that I would make dolmades or maybe try to braise them like you would collard greens. Unfortunately, I have no idea why kind of grape fine it was. All I know is that the grapes were purple and the leaves were green.
When the leaves were raw, they tasted fine and seemed edible. After blanching them, they became extremely tough, unchewable. When I tried to eat one, it felt like chewing paper--my teeth couldn't bite through it, it just mashed into a tight ball in my mouth.
I even tried braising some of the leaves for an hour or two and they were still unchewable.
This might make you feel as silly as I did when I experienced the very same thing. The grape leaves were from the fruit -bearing vines.
Since I only used the jar-brined variety, I didn't know that only MALE grape leaves are edible! The ones that do NOT bear the grapes. Yes, yes. I know. Who'd-a-thunk? In the 50 years of my grape-stuffing Lebanese heritage, I did NOT know that fresh grape leaves MUST be the male non-fruit bearing variety. The female leaves are like fabric. Just as you described! Unbelievably tough and abrasive. Maybe they're trying to tell us something...lol. Femme fatale... woman's lib in GREEN. Anyway. There's part of your answer!
I use only canned grape leaves. I believe the salt brine tenderizes them over time. Using fresh grape leaves means you need to remove the stems and tough veins and they need to cook for 30 - 40 minutes about half covered with liquid. But, even then, they may not be as tender as you might like.