A brave but cruel knight named Vadalon received Sir Ector de Maris' seat at the Round Table after Lancelot and his supporters left Arthur. Maybe this knight can be identified with the King Vadalon who banded Agrippe's daughter, but think it not improbable.
He besieged Mabon Rock, ruled by King Agrippe, but his army was destroyed when Agrippe’s daughter poisoned its water supply. In revenge, Vadalon imprisoned the girl in a set of iron bands, which caused her extreme pain until she was freed by Sir Bors.
King Vadalon could also be called "cruel", and, while I have not discovered whether or not Sir Bors encountered King Vadalon during the year he was pledged to carry the shield Agrippe's daughter gave him, or what the outcome of that fight - if it ever took place - might have been, it would as neatly fit the pattern of such events if Bors conquered and converted Vadalon and brought him to Arthur as a faithful vassal, eventually to be given a place at the Round Table, as it would if Bors conquered and killed King Vadalon.