Actually,Sonny died before Vincent was born,so he really didn't have anything but the mythology to go on. From the script:

MARY
What was Sonny like?
VINCENT
He was the Prince of the city. Yeah -- he died before I was born but I heard a million stories about him, he was a legend.

Vincent also tells Mary that her father was a hero and that "he saved the Family".
In Vince's world,regardless of the truth,Sonny and Uncle Mike could do no wrong.
As we see,Vincent was not much more than an undisciplined thug when he got into the mix,so he wouldn't have understood or appreciated the nuances of the Corleones in the context of the Mafia as a whole.
As time went on,he was schooled by Mike,and really came a long way toward becoming a decent Boss.
However in his mind,all of the negatives that you correctly pointed out concerning Sonny would have been selling points for a young cugine like Mancini.