You say you don't need the lens for either wide-angle or telephoto. I guess then on the 24-70 you don't need the 24-35 part, and maybe not the 70 end either if you get the 70-200 (or if you just crop a little bit in post).
I think the wider aperture would be more valuable than VC. I find I shoot in bad light a lot, and if I didn't have a 1.4 or 1.8 prime to use, I would have to jack up the ISO so high the noise and lack of detail would make me miserable.
Also, as for the lower shutter speed you can get with VC, two things:
1.). The f/1.4 prime lens is 2 stops faster than the f/2.8 lens, so that offsets the probably 2-3 stops you could realistically expect to gain from VC, even if it claims 3-4 stops.
2.). Vibration control is much less helpful or important at 50mm than it would be at a long focal length like 400mm. I very rarely am able to shoot at anything slower than 1/50th because I usually have moving subjects. VC only helps you with camera shake blur. It does nothing to help you with subject motion blur.
On top of this, you get the use of a much shallower depth of field with the prime, for when you want that for creative reasons.
On top of that, with the Sigma 1.4 prime,you get a lens that is considerably sharper. It is probably just a little smaller too, though not tremendously so.
I have the Sigma Art 35, and that thing lives on my camera. I thought i would always want a zoom, but i dont miss it. I actually enjoy how the fixed 35mm lens makes me move myself into position to shoot, and it makes me get a lot more pleasing framing in the process, compared to just zooming in and out with a zoom.
It really makes beautiful colors and super sharp images. Supposedly the 50 is even a little more impressive.