Umm, variant vampire template?
It's a lot weaker than a true vampire. I'd say a regular non-template monster, using the Vampire Spawn as a foundation.
Take the Vampire Spawn, Increase Strength to 18, maybe reduce its Dex or NA since it's only got AD&D AC7, although I would think we should aim for something a tad better than Armour Class 13. Drop the Energy Drain (I'd let it keep Blood Drain though), Damage Reduction and Gaseous Form, Add a reduced form of the Template Vamp's Children of the Night (1d3 rat swarms?) and add a 'skin trick' SQ.
Upon reflection, I'd keep the Vamp Spawn's +3 natural armour and +2 Dex, that's still pretty weak for a 6HD Undead, I could even see tweaking it up to a ghast's +4 NA.
So, 6 Hit Dice, Natural Armour +3, Str 18, Dex 14, Con -, Int 13, Wis 13, Cha 14.
Now, I'm guessing the 'skin trick' will let it masquerade as a living creature, but is it the Tyerkow's original skin, or does it steal the skin of a victim?
There's no mention as to how they propagate, make them Create Spawn from creatures they've blood-drained to death?