Well to reply to the original post, you don't beat him. Blender lords will butcher almost anything in front of them, especially the cavalry versions. So just kill the unit around him. I've had and seen more vampires die from CR by a long shot than were killed in close combat... ok except against killing blow dark elf spearmen. I am cursed against those damned things.
As for taking an old blood against him, I'd just say no. I'm not sure what kinds of lords you're facing but my vampire lords have slaughtered anything put in front of them. In the discussion above there are a few holes.
First, none of the calculations take into account re-rolling to hit nor additional attacks from wounds. Now the second isn't that big of a deal in challenges, but the first means there aren't three hits but much closer to all five.
Second, a lot of Lords are carrying the Other Tricksters Shard now. That knocks that ward save down from 50% to 25%.
Third, Beguile is becoming somewhat popular among some vampires (make a LD check at -3 or re-roll successful hits against the vampire). Inside of the Slann's range that still tolerable, outside it drops to a Ld 5 roll probably with no re-roll for BSB which is only around a 50/50 shot. Miss it and the old blood isn't doing a lot that round.
Lastly, consider the Vampire lore and specifically Hellish Vigor. If the lord gets it, the old blood is pretty much done in two combat rounds. Before people say "I have a slann" take two things into account: at 2500 pts you won't have a completely tricked out Slann and the old blood, he'll be close enough (becalming) but not maxed out; the Mortis Engine adds +2 to all Vampire lore casters so a Master Necro can still get targeted Hellish Vigor off fairly reliably even under the effects of Becalming.
So lets look at this again:
- The Vampire Lord attacks with ASF, hitting on three's with a re-roll resulting in roughly 4.44 hits.
- Wounding on three's will net 2.96 wounds.
- 5+ armor save bring it to 1.975
- After ward saves it's about 1.48
- A more realistic look at it would shake out to around two wounds one round and one the next. The old blood will have a damned hard time surviving a two wound round because as soon a two wounds go through that's two more attacks with the odds above.
In short, feed the damned thing a champion and kill the unit around him. It might cost you a few saurus, but the old blood will drop 3-4 ghouls/skeletons/what ever and help tip the CR difference to the "wiping out ranks at a time" level.
Pyre