What role did Victor Emmanuel II play in Italian Unification?
He had a part alright but net-net he was driven by individual selfish need of a Monarch, expanding his territory. This is the same kingdom (his fathers to be exact) that was opposed to the Carbonari, and "La Giovane Italia", and sentence Garibaldi to death in absentia. My opinion is while history will see him as a King that plaid in the unification, I see him as a selfish Monarch that will do whatever it take to expand his domain.
While the key leaders to the Unification of Italy were: Cavour, Garibaldi and Mazzini, Victor Emanuel played the role of an enabler as the King of the Kingdom of Savoy and a center point for agitators to unify around. He played the key figure head, for Italian Nationalist to unify around regardless of their political position - Monarchist, Republicans etc.
Also he was a key conduit and figure head for the communication and pacts that Cavour was concocting with Napoleon III - King to Emperor communication.
For example it was Victor Emanuel who accepted the results of the armistice of Villafranca, concluded in early July between France and Austria without consultation with Sardinia to the chagrin of Camillo Cavour. Victor Emanuel accepted what the French negotiated, a much reduced outcome than the Savoy-France pact promised - the territory of Lombardy.
He was also the King who that transferred Nice and Savoy to France forever given away Garibaldi birthplace a key architect of Unified Italy and a republican... The irony Italy is now a republic...