Can you create an algorithm which checks at a specific round (for example 50, 100, 150 and so on) if any VC is holding at minimum one settlement in Sylvania. If not, a specific amount of VC armies will be spawned as an event in this location (amount of armies depending on round). Next step another algorithm checks if Manfred and/ or Vlad are still alive, if one or both and their factions are dead, one of them will be leading the newly spawned armies (50% Vlad /50% Manfred when both are dead).
Same could be done with other evil factions but without coming back LL, instead only generic lords leading them.
For example skavenblight is not held by skaven, respawn several armies of a random wiped out skaven faction nearby.
If Grimgors starting settlement + nearby settlements are not in control of GS, spawn a random wiped out GS faction nearby.
And so on.
Or instead of simple respawning a more subtle way of a Dillemma chain will be initiated, where at first comes a bad omen dilemma with vampiric corruption +5 in a handfull random provinces. Than some random dilemmas of "something is found" pay the scouts and led to an end of spawning different revolting vampire factions on different locations with Vlad among them.