Vogel is definitely in with the brain surgeon but I don't think she's been behind the killings. My best guess was that she taught her son how to become a serial killer and since she was interested in studying brains she taught him how to perform brain surgery. Something got fucked up and he was sent away. So she started working with other psychopaths. No need to elaborate more than this since it's either too predictable or there going to give us some stupid big curveball that makes no sense.
This season has been better but the last few seasons overall have been pretty painful. After trinity they should have created a 5th season and maybe a 6th season combining the shit with Deb finding Dexter out and the real truth behind Harry.
Seriously though what point does Harrison serve in this show? Why wasn't he just killed off? He's basically a glorified neglected prop. I'll give him this though, he's better than Linden from the Killing.