Okay, first of all.
About the vampire creating hundreds of Spawn and going off to create lots of more Spawn and so on and so on...
I ask you this: How is this vampire going to Feed all of these spawn? People have been posing the question about a simple Vampire having to drain all this con from people to survive, how the heck does that same vampire feed 80 spawn?
There was a comic, I don't know what it is, but the vampires don't create other vampires; after feeding, they behead and burn the corpse. Simply because, more vampires means more competition for Food. If humans are Food, you don't want more of your kind getting there before you.
The more spawn you have, needing food, the more likely you are to rouse some suspicions by clerics and other nasty things, that will swoop in and stomp you AND your undead minons' butts. So, it's best to Lay Low. Even moreso, those spawn have to sleep during the day. Where are you stowing these guys? Hundreds of stackable coffins in a portable hole? When the sun comes, and you're on your way to the next village, *POOF* There goes your army. And, let's not forget a river's in the path, or you've... dare I say, 'got the shaft'.
Equally, I don't see How a vampire is going to go to every house in the village, knock on the door, say 'Hi, let me in', and proceed to eat the entire village before daybreak, THEN bury the entire village, to create his spawn. That seems a little... Work Intensive. First, he has to be invited. Then, he has to dominate each person to keep them from screaming. Then, he has to drain them, and kill them, do this 79 more times, and then bury 80 people.
Let us not forget that a thorp of 80 is going to have lots of children of varying ages, and people of considerable ages. I don't care if it's a spawn, a 3 year old doesn't make a good spawn, or a 70 year old grandma. And, you don't want to Feed off the old, either, since well... they're old, and diseased, or Weak. That's like eating a cold Big Mac when there's an 18 year old, powerful, full blooded prime rib down the street.
Really, the best tactic would be to get you some Servants, and either bleed them into a bowl now and then, instead of directly leeching off of them, or just dominate many able bodied men, monsters, and so on, have Them go ransack you a village, humanoid tribe, whateve, and get you your food.
Now, Secondly...
In reguards to the 'Slam' draining levels, okay, let us use a different varient power suggested here. Vampire drains +1s from weapons, or whatever. If a vampire had *this* ability, we all would complain on how that makes no sense and how it's not in the lore. But it does the Very Same Thing as the level drain: screws up the party.
The meaning Behind the level drain is the negative energy so infused in a vampire is brought outward by a violent thrust of power, which happens to be delivered by his Fist.
How about, for the sake of arguement, we picture the vampire grabbing someone by the skull, and Pouring negative energy into them. Yes, this could be 'Grappling', and it also could be 'Domination', but for the sake of arguement, this could qualify as a slam.
Next, let us look at the 'Con' drain due to bite.
Have you ever tried to grapple in melee? Especially when there's 3 other people smacking on you to Get Off? It's HARD! Even if you use that Domination ability, that doesn't work much in combat, because using a spell like draws an AoO, and that leaves you quite open. Limiting the level drain to bite really makes that kinda hard, to get it off. You can slam one PC a round. You can't bite one a round (But you can damn sure nearly kill ONE if you latch on there long enough).
Fourth, about Con being permenant or temp... The example was used that someone 30 years later still has Con loss... That's not a good example. If someone who lost permenant Con is alive for 30 years prior, then I want to shake that person's hand. They sustained cold winters, hot summers, Disease, and other Fort-save-dependant things, with a weakened fort and low HPs. If this person is a commoner, that's *amazing*. Most people who sustain Con damage usually DIE, if not from the con loss or the vampire sucking them dry, then by natural causes because A) Immune system weak, or B) Physical exertion and regular life takes it's tole.
Frankly, Other permenant con damage like Poisons, I think, is a lot more nasty then a single vampire bite. I'm more worried about Rogues with a big budget buying lots of Con poison, as aposed to a single vampire who Might bite me.
Lastly, to answer the proposed thing about vampires Feeding, they *could* drink from animals, y'know. I bet Cows have a Lot more blood. Riding dogs have average Con 15, as aposed to the human's 10. Horses have 17 or 15, respectively. True, it's not as tasty as a human's, but when you need some, and don't want to deplete the local population, go for it.
The vampire is just another example of monsters who Can be used for pure combat, and can be a Lot more. Mindflayers, Rakshasas, even Liches, these creatures are often more likely to work from the shadows, through a great network of spies, henchmen, and aliases. They may Never get their hands dirty. Professor Moriarity, until he came to meet Sherlock Holmes on equal footing in Sherlock's home, had never been seen, yet he was the head man behind all of London's crime. Equally, when he met Sherlock, it was a polite warning to stay away. There was no fight, there was no this or that. As so, can your creature.
They have the potential to be combat monkies, monsters, fodder, and what have you, but in that same light, they can be what you want. Mindflayers are either nasty brainsucking critters that try to just kill you, or they set up powerstrings, ala Speaker of Dreams, and may be behind empires, bandit bands, or secret societies even. So, it relies as much on you, as it does your monster.