Why was Vibhishana discredited in spite of having supported Lord Rama?

Vibhishana is a unique example of someone who was caught in a dilemma whether to support righteousness (dharma) or his people. He chose to renounce his brother in favour of dharma.
See, from all these stories, take only that which you need. What is the point of arguing for or against? If there is something in the lives of Vibhishana, Rama or Ravana that inspires you, take it, and if it does not fit in, or appeal to you, then you have learnt something from that as well.
Some people have asked me, why did Sri Rama send Sita to the forest based on a washer man’s opinion? Isn’t that injustice? I say, ‘Absolutely, he did not do the Art of Living course, otherwise I would have told him not to be a football of other people’s opinions!’ 
Kuchh to log kahenge, logon ka kaam hai kehna’ (People are bound to say something, that is what they do). Some might support Rama’s decision that he did it to uphold honour.
You can find some flaw or the other in every incarnation (avatar). At least I see faults in them, particularly in Parashuram. If one considers Parashuram God, then there is no one left you cannot call God. I find every human being better than him.
‘Don’t try to find fault with anyone,’ even Lord Krishna has said. If you start looking for flaws in Krishna, you will find so many that you might think you are better than him.
Lord Krishna told Arjun, ‘you are anasuya, (meaning one who does not look for faults in others) and that is why I am giving you knowledge.’ Although knowledge is given to everyone, the deepest mysteries are revealed only to those who are deserving and worthy. 
When discussing Vibhishana look at what qualities he has. People have the tendency to denounce others. Jains have condemned Lord Krishna to hell. According to them, Krishna will have to come back in the next period as the first Tirthankara. These are just concepts, don’t get into them. What you need to observe, understand and learn, and what is useful to your life, is what you need to extract.
There is a beautiful story in Bhagavatam. From the dirt in Lord Vishnu’s ear appeared two demons Madhu and Kaitabha. Lord Vishnu kept battling with them for a thousand years and he could not defeat them. He became exhausted and finally took refuge in Devi (Mother Divine). The Devi took the demons into the water and conquered them.
Now you see, this story is so pregnant with meaning. Madhu means craving and Kaitabha means aversion; now where do these arise? In our ears when we do not listen attentively. Just as there is flaw in our sight, there is a fault in our listening. Someone may be paying you compliments but you will think that they are irritating you, playing with your emotions. Someone incurred losses in the share market, and an acquaintance remarks, ‘Oh you look good!’, and this person thinks, ‘What a hypocrite, he is telling me I look good when I am miserable.’ And anger and enmity begins because we did not hear properly.
Cravings and aversions happen through listening and this cannot be eliminated by us. So inner-strength (the Devi shakti) is awakened, and these negative emotions are destroyed through love; water signifies love. This is the secret of this story.
In all these stories there is a spiritual aspect that we need to recognize. It is not written anywhere but meditate on it, ponder over it, and you will realize, ‘Oh, so this is it!’