They get back to Mia's house, dance trophy in hand, and Mia puts on some tunes and starts singing and dancing while Vincent goes to the bathroom.
In the bathroom, Vincent gives himself a pep talk about just going home and not messing around with Mia.
Meanwhile, Mia finds Vincent's heroin but mistakes it for cocaine.
She snorts it and goes into shock. Snorting heroin actually brings it into the bloodstream faster than injecting it directly into the bloodstream with a needle. It's generally a very bad idea.