Complex sentence: A favourite of John's, Vieuxpont was one of only a few lords loyal to him in northern England, whose inhabitants became so discontented with the king's rule that they eventually rebelled.
Split into simpler sentences: A favourite of John's, Vieuxpont was one of only a few lords loyal to John in northern England. Inhabitants of the area were discontented with the king's rule to such an extent that they eventually rebelled.