Jonny deserved more credit.
I think Jonny was the strongest character in the movie. He was definitely naiive, but not stupid. He saw things as they were. But his lack of the courage to stand firmly within himself and lay anything on the line gave him a sort of "arrested developement". He never gave himself the opportunity to really find out what he was made of out of a fear of falling short. And yeah, he was whiny and judgemental, but he was objective about it; though maybe not in regard to himself.
But I don't think Jonny pushing Vinny on the dance floor was really out of anger toward Vinny working up on "his girl". He was lashing out due to his frustration with not having the balls to gain what he wanted for himself; let alone having it scooped out from under him by a scumbag like Vinny. He knew he was in the wrong, because he had no claim to that girl (he didn't even really know her) outside of the ways in which he deluded himself. It was a moment of clarity for him. Just as him seeing her in that porno revealed to him that his perception of her was more than idealized.
I think this movie is overall a story about the possible personal growth that being in anykind of group of friends can afford an individual. All the characters knew what their flaws were, but they just told themselves and eachother whatever they had to in order to make it through the day. Which is the point of the movie. A real homeboy is someone who you can truthfully divulge all aspects of yourself to regardless of what harsh criticism or emotional encumbrance they happen to reciprocate, in turn. 'Cause you know they're gonna be there at the end of the day. But it's ultimately up to oneself to utilize that dependability not as a crutch, but an illumination of the undistorted truths it can reveal about themselves. Jonny was pretty on the spot in that regard, and I related to his character in a lot of ways. This is definitely one of the most profound and enlightening stories of friendship ever written. A true gem that deserves more praise, in my opinion.
"Aint you never been to the theatre before?" - Bill the Butcher