So I began testing. I chose Gin-Wulm (The Best Defense smith) as a companion. I didn't know at the time he's one of the toughest named NPCs in the game at level 30. That's good, because I might be encountering some impossible odds (I was level 2).
Testing proved to be great fun. Skeleton Heroes and Goblin Warlords gave my companion and me some long, exciting battles. But then I visited a den of vampires. I'd already given them some of the best levelled lists from my extensively revamped list design, thinking they should be ancient and well-armed. Unfortunately, Gin-Wulm was killing them in two or three hits. So I made them tougher. Still, he was killing them in two or three hits. I should note that each time I went back in to test, it was from a save just outside the cave which I hadn't visited before, so there's no chance the game was remembering earlier (weaker) iterations of the vampires.
First I made the vampires level offset 24-30 with a Calc Min of 18 (meaning they were 18 at this point). Two hit kills for Gin-Wulm.
Then I made them straight levels 24-18 (no offset). Again the Argonian cleaned the floor with them.
Finally I unchecked auto calc stats and gave each type of vampire between 300 and 500 hp. Gin-Wulm was STILL wasting them with ease.
I took a look at some of the other creatures being fought in that dungeon. He fought a headless zombie and a Skeleton Guardian at one point. Those creatures have 175 and 170 hp respectively (No PC Level Offset for those ones). Gin-Wulm beat them, of course, but it took several hits and the fight lasted at least half as long as the battle song.
Can someone tell me what I might be missing? I would really like to make vampires some of the toughest, most feared creatures in the game. As it stands now, they would certainly devour my level 2 testing character (and did), but as I get higher, or when I bring high level friends, these 300-500 hp NPCs won't even offer as much of a challenge as a 175 hp creature.