Vance Dennis is the ex-boyfriend of recovered drug addict Tricia Knowles. After she had a son, Austin, she sold him out and got him imprisoned on drug charges. Out of revenge, he sent the Murphy Twins, two twin drug dealers and mercenaries, to overdose Tricia and kill her.
Her mother Sandra Knowles never forgave an undercover Fin for not saving her, ad he went to interrogate Dennis in prison. He didn't admit to the murder, but an informant of Mike Sandoval confirmed the truth.
Dennis' charges were added to with murder for hire, and his sentence was changed to life without parole. (SVU: "Haunted")