Welcome, Brickman.
The short answer is: filmic license. It made for greater drama. We expect that two Mob hitmen on a mission would empty their guns into their victim. But, Vito had to survive the shooting. Logic be damned. Arguably, a combination of Vito's heavy overcoat and layers of clothing, combined with sheer luck, might have allowed him to survive all those shots. Not likely, but the imperative was for FFC to produce a dramatic scene, and for Vito to survive.
And, you're right: there are plenty of illogical shooting techniques for the sake of drama. Why did Rocco and his associate need to use a pair of Madsden submachine guns to kill Tattaglia and the hooker he was in bed with from a distance of about five feet? Why would one guy (Neri), armed with just one six-shot revolver, be entrusted on a mission to kill (as it turned out) Barzini, a bodyguard and the chauffeur? Awfully thin margin. And his .38 Police Special is neither particularly powerful nor accurate at the distance he shot Barzini. Why did Clemenza need a shotgun (big, heavy, conspicuous) to whack Stracci and the other guy in the elevator, when a handgun would have been less conspicuous, lighter and easier on a fat old guy climbing stairs? My favorite anomaly in GF is FFC's homage to Arthur Penn ("Bonnie and Clyde") when he has Sonny riddled with about 100 rounds from Thompson submachine guns--and still able to stand, writhe, scream, etc. Tommyguns fired .45 ACP rounds--the proverbial "big, slow round." They are designed for knockdown power. Even one hit would have knocked Sonny to the ground.
Why? Drama! Visual impact! Fodder for people like you and me to post on.