Vibishana can't able to choose neutral because of boon by Brahma.
Brahma then addressed Vibhishana, 'O my son, I am much pleased with thee! Ask any boon thou pleasest!' Thereupon, Vibhishana replied, 'Even in great danger, may I never swerve from the path of righteousness, and though ignorant, may I, O adorable Sire, be illumined with the light of divine knowledge!' And Brahma replied, 'O scourge of thy enemies, as thy soul inclines not to unrighteousness although born in the Rakshasa race, I grant thee immortality!'
[SECTION CCLXXIII, Draupadi-harana Parva, Vana Parva, The Mahabharata ]
You can notice about the boon from Valmiki Ramayana also
Thereat the virtuous Bibhishana spoke with joined hands, — 'O reverend one, since the spiritual preceptor of all creatures himself (is pleased) with me,— I (deem myself) as furnished with every perfection, even as the Moon is environed with rays. If thou wilt bestow on me a boon with pleasure, then, thou of excellent vows, hear of the boon that I would have. May my mind remain fast fixed on righteousness, even when i shall happen to fall into high peril ; and may I attain Brahma knowledge without any instructions ! And may every sense of mine sprung during the observance of particular modes of life, be in unison with righteousness, — so that I may practise the form of religion (in harmony with any particular mode of life !) O exceedingly noble one, this best of boons, is even what is sought for by me, for nothing in this world is incapable of being attained by those attached to righteous- ness.' Then the lord of creatures, being again delighted, spoke unto Bibhishana, — 'As thou art virtuous, so all this shall come to pass on thy behalf. And inasmuch as in spite of thy having been born in the Rakshasa race, thy thoughts, O destroyer of enemies, do not originate in sin, I confer on the immortality
Since Ravana is unrighteousness in abducting Sita. Vibhishana can't be inclined towards Ravana. Hence he can't be neutral and selected righteousness Rama.
