I think the clear plastic rear window is most susceptible to the cold and salt, it can get brittle and crack. That is why I prefer the glass rear window with convertibles.
The vinyl top, I think it can hold up okay to the salt, but the salt will still wear it out over time. However I haven't tested that as I don't want to risk ruining my top. lol
As far as snow, not letting it pile up and weighing the top down is important. When I park my car for the winter its outside, and I carefully brush the top off with a soft brush after every snow fall.
With cold I think its important not to open the top, the material shrinks in the cold and you don't want it to shrink while open then risk stretching it or tearing it when trying to close it. (I've seen convertibles open in the winter. They must like the cold. lol).
