The root "wand", as I recall, means "white". There is a Whitehaven in Cumberland and a Whitby in Yorkshire. Glennie lists a Caer Vandwy and identifies it as Cramond, near Leith, on the Firth of Forth, Scotland. Whitehaven seems too near King Pellam's territory if that is identified with the Lake District. I would prefer to identify Wandesborow with Caer Vandwy.
It may well be, of course, that Wandesborow and Vandaliors, which see, should be considered identical, in which case the Cornish site would be taken. It does not seem impossible, though, that the rebel kings were simultaneously or coincidentally attacked by two different foreign enemies at two different strongholds.