I think mine is when he kills Finucci - in fact young Vito's interaction with Finucci is his definining moment - and the effort and even style he puts into killing Finucci - watching him stock the don on the rooftops of NY, it was almost an artform.

(I was thiking of that scene when i saw Vincent kill Joey Zaza in Part 3. A death is still a death, but Vincent kills Joey with brute force, while Vito kills Finucci with style. Does that make sense?)