There's pretty much only one thing you can do to get rid of a vampire lord in a fully ranked unit. It won't be easy to pull off but if you can do it the vampire will be very limited in his options.
Right, first you need two damsels with two dispel scrolls each. Hide them away somewhere safe if you can. Next you need to get two units of fully ranked knights of the realm with a paladin in each (one
BSB and one combat paladin*, arm the combat paladin with something that will continue to increase his stats in a second round of combat, ie. not a lance) and as many combat res increasing doo dahs as you can.
Charge both units simultaneously into the vampire lord's unit... Needless to say this is the tricky part as not only do you have to get the charge with your knights, you also need to find a way to protect their flanks (perhaps mounted yeomen positioned to re-direct charges). Good luck with this step, it really is crucial that you pull it off.
If the vampire or unit champion declares a challenge, accept with your own champions, not the paladins. Direct ALL available attacks against the unit, even grave guard will have a hard time against that. Now in an ideal world, the combat resolution will kill off all the remaining unit, vampire included... but this isn't an ideal world which is why you got the damsels earlier. In the vampire's magic phase he will inevitably do everything he can to repair his unit, don't be afraid to use all 4 dispel scrolls to prevent this from happening.
Now it's that moment in the game that all Knights dread... the second round of combat. Luckily, you should be facing a mere fraction of the lord's bodyguard and you have a vastly superior fixed combat res. Your combat paladin comes into his own here, as it will be him you rely on if you're facing something tough like graveguard. Again, direct all your attacks against whatever's left of the unit. By this point, you don't want there to be any more than five bodyguard after your attacks. Unless you're fighting a truly monstrous combat lord (who should be on a dragon anyway instead of in a unit) you should beat down the remaining bodyguard AND the vampire with your combat res. Huzzah! Croissants for everyone!
I'm aware that this tactic is incredibly difficult to pull off and personally I think there's a better one... ignore the lord's unit and kill everything else. After all, vampires are considered immortal for a reason. Assassination units (lord on hippogriff etc) can potentially work... but a more likely outcome is the vampire lord will whip out his big sack of dirty tricks and obliterate the would be assassin. For this reason it's crucial to whittle down the bodyguard until you know what the vampire is capable of.
*A lord can be used instead of a paladin, but personally speaking I'd want to put my general in a unit of grail knights and go smooshing the rest of the vampire army.