Good luck with that one. I've asked this in here a couple of times for a fresh Welch's grape juice flavor and didn't get a nary a response. Like you, most grapes taste like the soda or candy that I've tried, which I admit have only included 2 or 3 companies. I have found it interesting that grape is often left out of most vendors' juice lineup. Not sure if that is a reflection that it is a difficult flavor to duplicate or what.
The Classic Grape from Smokeless Image comes close, surprisingly, but the flavor is admittingly weak. I do like it, it's just not exactly what I am looking for. I found that adding a little menthol seems to liven the flavor a notch.
Someone had responded to one of my requests claiming that Ritchie's had a grape that tasted like the fruit juice. I found a couple of places who sold the brand, an Italian company, but they never offer the grape flavor.