So here's the problem, I have an entire faction set on very aggressive, and the Player Faction is neutral towards this faction. So the NPCs in theory SHOULD be hostile to the player until the player joins their faction. But nothing seems to make them attack the player. They are also set to help friends and allies, but won't defend each other if the player attacks them. Further more the daedra and undead NPCs even though they are set to the same faction, and not set to frenzy aggression, STILL attack the NPCs in their own faction, and even each other. As always I'm using custom IDs for every NPC I created so it's not due to a mod conflict. I'll provide some screenshots in case I'm doing something obviously wrong and missed it.

http://i.imgur.com/Ri8exi2.png

http://i.imgur.com/MgUfOY1.png

http://i.imgur.com/3xSaFP7.png

Any idea why the NPCs aggression isn't working properly? I'm just baffled and I'd be very thankful for any suggessions on how to solve this.

Did you set the faction relations both ways? I don't know if this holds true for Skrim, but in older Beth games, you would have to set the interfacing relationship for faction B with faction A AND faction A with faction B - meaning you might need to add your vampire faction to the player faction (or even better since you don't really want to make a change like that to a vanilla asset) you want to create a new faction for the player and add him to it and use that for the interaction relationships between the vampires and the new player's faction.

For some reason NPCs that are set on very aggressive attack their own faction, even though the official page claims only frenzied NPCs attack their own faction. And also well, there's a dilemma here: There are three main factions in this mod, all three are supposed to be hostile to the player by default, and drinking a potion is supposed to make only one of the three stop being hostile to the player.

I did make a duplicate player faction as you suggested, though the NPCs still attack their own faction if set to very aggressive, yet only attack the player if set to very aggressive, and since they attack their own faction even if the script is right they'll still attack the player.