Moved from Gall Midges.
Whoops... I should have said, there was a larva inside... I hadn't looked at it under magnification. Turns out this is a moth gall rather than a midge gall.

Moved from Unidentified Galls.
I found a bunch of these today and opened one; there was in fact a midge larva inside.

Moved from Miscellaneous Grape Midge Galls.
Well, based on this paper, the grape-feeding midges I mentioned below are now placed in the genus Vitisiella, but these don't look quite like any galls I've seen illustrated or described, so I'm moving them back to 'unidentified galls' for now, even though I still think they most likely are midge galls.

Moved from Unidentified Midge Galls.

Moved from Gall Midges.

Moved from galls.

This book attributes "irregular, succulent swellings on various plants parts" of grape to "Dasineura spp., Janetiella spp., and miscellaneous cecidomyiids . . . the species responsible for the entire suite of these galls on grape form a monophyletic group. A separate genus will eventually be erected for them. It is now very difficult to associate any of the described species with particular galls. More than one species has been reared from one kind of gall."

that it may be Grape Phylloxera. Interesting that it only occurs on the larger leaf veins
I did some more research and I also wonder about Gouty Vein gall Midge: Continaria negundinis