Bisping was responsible for 17 of the 24 months the MW belt went without being legitimately defended. In that time, Whitaker beat Natal, Brunson, Jacare and Romero (and Uriah Hall who was coming off the Mousasi win). When Whitaker beat Romero, it had been 575 days (19 months) since the MW belt had been defended against a top contender while Whitaker had knocked off the #12, #6, #2 and #1. Whitakers belt was the real one. A title that isnt legitimately contested for over a year loses its legitimacy, but when they gave the real champ a belt for beating the top contenders, Bisbings claim to the title became the biggest joke in the sport. It would be like GSP claiming he was still the WW champ today.....
Calling Bisping undisputed champ after July 2017 is ridiculous. He hadnt legitimately defended the belt since the fLuke over a year prior and there was a champ in the division who just beat the #2 and #1 contenders back to back. Thats about as disputed as a title can possibly get. Only one belt had any legitimacy....and it sure as hell wasnt the ornament that Bispieng carried around.
The belt wasnt vacated, it died of illegitimacy after going 2 whole years (726 days) without being defended against any top MWs.