In the last part of the film, Vincent criticizes Max for being trapped by the routine of his life and being unable to adapt or change for the better. Vincent has to seriously cajole Max to hire himself out for the night for a huge payday. Max never actually verbally agrees, but does go along with it, showing that he has a spark of rebellion. In the final confrontation, Vincent fails to kill Max because of his routine reflex of performing a chest-chest-head tap. He attempts to shoot Max through the subway door, but ends up hitting the steel beam located between the two windows.
Getting shot in the ear does in fact result in losing the balance more often and finding it harder to concentrate - at best. Exemplified by Vincent getting subtly clumsier after suffering such a wound (tripping over a chair he himself threw through a window, getting stuck between two closing subway doors, taking longer to react and make choices, etc).
Executing five people in one night, one of whom is a U.S attorney is a particularly difficult job and must command a suitably high fee for the effort. In fact so dangerous that it might even be the last job for Vincent. Part of Vincent's determination to see this through is that he needs to get away, then get paid if his cover blown this time and he can't work again. Meaning him taking such risks as killing a cop and opening fire in a crowded night club.
Even with the stress of his job, it's unusual for a guy as young as Vincent to have a full head of silver hair. It's well-known that hitmen alter their appearances in a variety of ways — wigs, glasses, facial cosmetics, hair dye — to build fake identities and obfuscate their real age.
