The whole thing is just simply one of DPC usual flip flop.
Quinn was supposed to be the Antagonist of the story and by doing that Hard drug scene in the beginning of episode 2, it was mean to show just how BAD she is, how dangerous etc. The clear indication of, Here is the Bad girl/ guy of the story
But, as DPC started to like Quinn more, he realize He can't make her that Bad, so he made a smal retcon to that storyline, and we learned that Quinn not doing this stuff, she just sell light drugs.
And when Vinny come back to the picture, He revisit this, by showing a needle to us again,
to ensure ,
" See? See? , how Bad Vinny is, he is really bad like so Bad, , He is the one who selling this shit to Tommy , and he has a Gun, and so see? , he is the BAD guy. I bet you did not see that one coming, huh? I think I will ask 40 bucks for this "
There isn't really anything more to this.
I mean some of you guys give more thought into this than DPC himself
.
It funny in a way, but the truth is that unlike a "certian theory maker" who believed that DPC has some grand master plan, the truth is he really doesn't have any. And as the game progress it's more and more visible, that he just goes with the flow, and made shits up as he develop, that is why he is unnable to contain himself when it's comes to episodes.