I bought a Gavia back in 1994 and know Pinarello's mid-90s line-up pretty well (as I lived near Treviso and was/am a big fan of Pinarello.
Vuelta and Stelvio are essentially the same bike. Oversize lugged steel. The Vuelta came first and uses Columbus EL/OS tubeset. The Stelvio came out when Pinarello started a big move towards using proprietary tubesets made for them by Dedaccai. So the Stelvio model uses Dedaccai's version of oversized steel tubes that they made under a Pinarello name called Kens. Essentially the same bike.
Radius also used Dedaccai tubeset made under a Pinarello name but was tig welded with the tubes flared into a square shape at the ends. Personnally I prefer the lugged the Vuelta and Stelvio models to the Radius. Much more elegant.
I don't know the exact relationship between Pinarello and Dedaccai, but it seems to be very close. About1995, Pinarello started moving towards all of their bikes being made from "Pinarello" tubesets made by Dedaccai.