This may have no bearing on the problem, but for what it's worth .....

Canoes made of Royalex (which I understand is basically vinyl) do the same thing in their later years. The recommended solvent is denatured alcohol. It removed the sticky nicely, and I followed it with a rubdown with Armorall, the milky-white stuff intended for leather and vinyl. So far, it has not recurred.

d.