Why is Vibhishana not considered a traitor?
In the most simple of contexts, Vibhishana is not considered a traitor because he recognizes the path of righteousness and dharma and moves towards it. Vibhishana is shown to be an absolute devotee of Lord Vishnu. Throughout his characterization, he is shown to only have this as his primary goal. When asked by Lord Brahma what boon he would like, the younger brother of Ravana makes it clear that to be a devotee of Lord Vishnu and to meditate in his name and for his darshan is a boon enough. In this, he is shown to be representative of righteousness and a sense of honor. When his brother kidnaps Sita, Vibhishana is stead fast in his opposition to it. He recognizes the intrinsically evil nature of his brother's actions. Vibhishana disobeys not his brother, as much as he breaks from the lack of dharma and lack of righteousness that is displayed. He is a traitor to the forces of evil and to a sense of lawlessness. This is where he is not considered to be a traitor. It is the genius of the scriptures like the Ramayana to show that individuals can be loyal to people, but ideas and actions are also where human loyalty can lie. In this, Vibhishana is not disloyal to that which is pure and good, reason enough why he joins forces with Lord Rama to defeat the evil nature that his brother represents.