It is all about how we see the situation and from what side.
yes, vibhishan is looked down upon and the famous 'ghar ka bhedi' is also used in a negative way.
All I can say is he was a Vishnu bhakt and was with the right. As wat is shown in the show, he already helped mandorari to find a solution in the form of givin info about amrit. Here only, he could have stepped back and could not have helped her bhabhi as he knows how his brother is. When the evil acquires a mammoth status, it has to be destroyed and vibhishan had to be become the medium. the ghar ka bhedi should not only be seen in a negative way, but also in a warning way, which means if u do wrong, nobody (not even your family) is gonna support you.
Goin by wat u r sayin, I can also say that i did not find mandodari act of bringing amrit and givin power to her husband right, just because she was a wife and her most important dharma was to save her cruel husband despite knowing d fact that wat destructions her act is gonna create. If u c vibhishan as a betrayer, i see mandodari as selfish in dat particular act.
In kuruskshetra, lots of such loyalties or righteous notions were broken and every sam-dam-dand-bheda was used even by Pandavas as directed by Lord Krishna to win and for the establishment of Dharma because the benefit of humanity was in pandavas win.