Does a VGT have a B-screw? My 1980 Superbe doesn't. Does the derailleur hit the cog with no chain and no cable, moving the derailleur by hand? If it does, you're probably hosed.

Different dropouts had different derailleur hanger lengths. I know that Suntours were longer than Campys. That same Superbe is max rated at a 23T, but I'm running it on a 14-26, and I'm pretty sure a 28 would clear the top pulley. That's on a Suntour dropout on a Fuji frame. I'd guess that your Rossin has Campys, but there were several models and I don't know if they all had the same hangers. I seem to think I remember that there was something like a "racing" version with a short hanger for faster shifting on small clusters, but that might be false memory. (If such a thing existed, I'd be unsurprised to find one on a Rossin... they did have a "make things weird for theoretical gains that would be marginal even if real" thing going on back when... part of what makes them cool bikes.)

If your dropouts have adjustment screws, try removing them and running the wheel all the way back. Often, this will get you that last bit o' clearance.

--Shannon