I really like Vermillion and she stays in my LO, but having a succubus hang out (unacknowledged) in the Riverwood Trader didn't make much sense to me. I decided to move her to the Misty Grove where you meet the Daedric Prince Sanguine when playing the A Night To Remember quest.
Why you may ask?
Sanguine is the Daedric Prince of lust, perversity, and unnatural sexual relations. So this felt like the perfect place to find a succubus follower. You will find Vermillion on the 3rd stone bridge just before entering the final area with the dinner table and Sanguines shrine. See mod images for a birds eye view of her placement. She can wander a little so she may be found walking around close to the bridge. She will follow you out when Sanguine teleports you to the Dead Man's Drink.
Make sure to add her as a follower (and that you have follower space to add her) before you enter the Misty Grove through the portal or talk to Sam Guevenne! You may lose out on obtaining her as a follower forever!
If you mess up and don't or can't recruit Vermillion before Sanguine teleports you out of the cell you can COC back in to recruit her. You'll need to COC to get back out or use a teleport mod (if you have one).
Code to get back into Misty Grove
COC DA14Interior
This is just a simple ESL-flagged esp. Make sure it loads after the VermillionSuccubusCompanion.esp file.

Vermilllion for the Vermillion the Succubus (Fully Custom Voiced Companion) mod.
Misc credits not pertinent to the mod
CoralineKoralina for the Vermillion Replacer by Koralina - COtR - ESL mod (seen in screenshots).
Daymarr for the COCO Succubus - CBBE-UUNP SE outfit mod (seen in screenshots).
Instructions
Download and place after Vermillion follower mod.
Vermillion the Succubus in Misty Grove
Changes the follower Vermillion The Succubus spawn from The Riverwood Trader to Sanguine's Misty Grove.