You are wrong, and you weirdly ignore literally everything the story tells us about Quinn, the drug, and Tommy.
She injects it either because she herself is either already an addict and won't mind, or as an effort to convince Tommy, in either case it doesn't lend any credence to her statement.
And Tommy doesn't smoke that much weed, we only see him smoking it a couple of times, we however know that he bought the heroin/morphine from Vinny, we see the syringe on his table in E9, which is the drug Quinn convinced him to use, the same drug he then had to buy from Vinny, similarly Heather tries to convince Quinn to give Tommy some extra so he won't get into her supply, we also see her passed out next to Tommy after using the same drug Quinn injected, but we never see Heather smoking weed, the implication being that Tommy is an avid user and addict of the opiate Quinn is providing.
And Vinny is more successful because he one the one hand overcharges his customers without them knowing about it, all while pushing them to buy more than they need, as is evidenced by Lily's dialogue with the Pink Rose girls. And Tommy may have bought some weed from Vinny, he smokes weed on the roof when MC goes up there with Zoey, but you are in part correct (and this is a subject I have never disagreed with you on), his primary purchase from Vinny is the heroin/morphine, which he was no longer buying from Quinn, but due to his addiction he had to find a new provider for, namely Vinny. This is the drug Quinn was referencing when she remarked about him turning up like an addict looking for his next fix, not weed. Quinn got Tommy addicted to an opiate, and he went to Vinny when he couldn't get it from Quinn any longer.