If Vibhishan is not in the Rama's side . What is a chance of Ravan to defeat Ram in the war. As we can see that Vibhishan has pretty good impact in the war. like;
1) He told Ram about real Makarakhsha's(Son of Khar) identity among all clones or fake Makarakhsha.
2) When Laxman(Ram's Brother) had severe injury cause by the dagger of Indrajeet(Son of Ravana) He gave the idea to consult Raj-Vaidhya(Royal Physician) of Lanka (I think his name is Shushurna) which gave the idea of Mrit-Sanjeevani (Some kind of shrub which has property to give life to person who is dead or about to die) which eventually save Laxman.
3) He told Ram about Indrajeet worshipping Nikumbla to get Chariot and If he ride that Chariot, He will be Invincible and hence Laxman with the help of vanaras failed Yagna in between.
4) He told Ram that Ravan has amrit in his belly that is why he cant be kill until you dry amrit from his belly.
This is very important information provided by Vibhishan to Ram. Is Vibishan is actually a cause for defeat of Ravana ?
Note : All information is gathered from Ramanand Sagar's Television Series Ramayan