As it happens, I have just been shooting with a Summarit 75f2.5 over the last couple of days. My friend Terry Cioni bought one and graciously lent it to me.
a/ Going from the CV 75f2.5 to the Summarit 75 you will see a slight improvement in wide open performance. Not staggering, mind you, but slightly better edges than with the VC.
b/ It is not a small lens! I expected it to be close to the VC in size, but it is actually closer to the Apo Summicron 75. I did put a shot of the four 75's on my Flickr site a couple of days ago and there you can see the difference.
c/ If I had a VC 75, I would be hard pressed to spent the $12-1300 for a Summarit 75. I think I would rather save up for a 75/2 Summicron instead! As Roger says, it is a spectacular lens. In my estimation, one of the best Leica lenses ever! It is particularly good in close up and wide-open. If you are used to the edginess of the 35/2 Asph, nothing will really satisfy you until you get the 75f2 Summicron. So a couple of month on "hambutger helper" and macaroni and cheese is rough, but you will have the best Leica lens made!
I know, this is not much help in trying to avoid the inevitable (the 75f2), but thats my opinion.
Of course, with the introduction of the C-Biogon 35/2.8 from Zeiss, I would suspect that we will see a 85f2.8 C-Sonnar soon enough (pure speculation though, but it does kind of fit) and with the quality of the current C-series, the 21/4.5, the 50f1.5 it could be a worthy contender and most likely outperform the Summitar 75!