While on Vulcan, the crew discovers (mostly through detective work by Dr. Leonard McCoy, using the aid of a sentient computer named "Moira" resident on the Enterprise) that the secession movement was sparked at least partially by T'Pring, Spock's childhood bride-mate, who has nursed a grudge against Kirk and Spock for years for the deception involved in their participation in the koon-ut-kalifee ceremony (described in the original series episode "Amok Time").

T'Pau makes the decision to transfer her katra (in a sense, her soul) to Amanda, instead of another Vulcan, proving her trust in certain members of the human race, and tells Kirk, Spock, McCoy, Sarek, and Amanda that since T'Pring's plot was able to thrive in secrecy and subterfuge, it should be countered with openness and honesty, and that the information should simply be given to the Vulcan population.

By this point, Kirk and McCoy (who were, with Spock, invited to participate in the debates as Starfleet representatives), have already given their arguments against secession.

Spock's argument concludes the debates, and the planet votes on the secession, which is decided by a large margin in favor of remaining in the Federation.

[1] Ann-Marie Cahill of BookRiot.com recommend the book as "a lovely display of the importance of cultural identity and heritage with the evolution of any society; a key concept of the Star Trek Universe.