Vram in and of itself is fairly useless, it's just a buffer who's minimum size is dictated by the resolution and detail of whatever game you are playing.
The "extra" memory cards - the GTX760 4Gb, GTX770 4Gb and GTX780 6Gb are all of very dubious merits for various reasons, some of which are unique to the individual models, others are shared.
Extra Vram is only useful if it can actually be used. Getting a weaker GPU with a higher amount of Vram is pointless because at the types of resolution+settings you'd need to exploit the extra memory, the GPU itself isn't strong enough.
On most of these models, the memory interface itself remains unchanged which often means that to produce a scenario where that memory can really be used is near to impossible.
There is also a kind of myth that "running out" of Vram makes things unplayable, when most benchmarks on games with high memory demands will actually show a more smooth decline in performance over a sudden drop off. There are exceptions to this - any modded game where things can be super unpredictable, and games which add arbitrary restrictions on settings based on your video memory (namely Rockstar games).
Finally, there is the cost. A 4Gb GTX760 could be more expensive than a 2Gb GTX770, and despite the extra memory there is nearly no scenario in which the GTX760 would outperform the GTX770. This is less significant at GTX770 level as there's a bigger gap to the obvious rivals (though the 280X and 290 do invalidate it's existence a little) but comes into play again at GTX780 level (with the GTX780Ti).
Less significant to you, but as a general statement - AMD chips have generally come with higher memory (and bandwidth) in the last couple of generations. If memory is a significant and legitimate concern, the AMD options are going to be incredibly tempting.