A script will need to go on the Activator (in an OnActivate() Event)
In the script/event you would need to check (if statements):
If the NPC using the activator is the PLAYER
If the PLAYER is in the correct Vampire Faction
If both of those were true, then you would allow the door to open.
Exactly how you go about it depends on how things are currently setup - Most importantly whether the Activator & Door combo are controlled by script, or by Activate-Parenting. So it's difficult for me to be precise with what you need to do.
Some examples:
How to make things work: http://www.creationkit.com/Complete_Example_Scripts#A_Fully_Functional_Dwemer_Button (this shows you how to setup a "Dwemer Button" to do things ... Your lever for your door would need a similar script)
Note: If you copied things from a warehouse cell, then you may well be currently using Parenting to activate ... you can read about that here: http://www.creationkit.com/Bethesda_Tutorial_Traps_and_Prefabs#Basic_Activation_Parenting
Detecting the Player http://www.creationkit.com/Complete_Example_Scripts#A_Trigger_That_Detects_When_The_Player_Enters (that is setup on a trigger box, but hopefully you see how you could use the same commands/syntax)
In a Faction: http://www.creationkit.com/IsInFaction_-_Actor (see the example ... only you would be testing the Player, not a guard)
Note that the examples assume you know about ALIASES & PROPERTIES for your quests
Note: If you are not very experienced at modding/scripting you may well find that doing the tutorial is well worth your while. It won't get this thing working, but it will allow you to start understanding how the whole thing hangs together. This is a very complex toy and it is little fun if you do not understand how it works behind the scenes. http://www.creationkit.com/Category:Tutorials (we - well, most of us - started off with Bendo!)
