My players are quite into the mystery of the disappearance of House Rogarvia and the 'event' of the Varnhold Vanishing will undoubtedly encourage them to draw the conclusion that the two phenomena are interconnected. And why not, as it seems exactly the thing the Oculus of Abbadon would be able to accomplish?
Following this line of thinking, there are a few possibilities. Firstly, that there is more than one oculus, which, however devaluates the importance of the artifact. Not in favor of this option. Secondly, the oculus has been stolen from Vordakai, and then was returned. How? Why? The events of VV presume that Vordakai is just re-emerging from his slumber, getting to know the world, etc. So either he has been awake for some time - enough time to get out to retrieve his oculus. While this plotline is a possibility, I'm going for something different altogether.
Suppose that someone did found out about the artifact, its history, and current location, and tried to steal it, only to awaken the lich in the process? And suppose that said person was Noleski Surtova (or a close family member or vassal of his). Two possible scenarios unfold: either Noleski blackmails Vordakai into helping him, e.g. by threatening to destroy his phylactery (which he retrieved how?), or Vordakai dominates Noleski, picks his brain on contemporary politics, and uses him as an intermediary to rule in his stead, while the lich rebuilds his strength.
Why doesn't Vordakai simply do his evil lich stuff and get rid of Noleski? Well, I thought of an interesting twist on the "atrophied lich" theme. Suppose Vordakai is the original Vordakai Cyclops Overlord who ruled before Earthfall, and not a minor student of his. Suppose he foresaw the cataclysmic event, and prepared, much like the Runelords, for his own survival. In my scenario, Vordakai, seated on his throne, which was hewn out of the stone foundations of the Tors of Levenies, melded with the mountains. Yet, as always, something went terribly wrong, and the wizard did not awake as planned. Instead he was slowly petrified into the rocks. He only survived because of the presence of the Oculus in his head.
So, when Vordakai awakes, he is a fully capable lich, probably a 20th level caster with some mythic tiers thrown in, but much of his spellcasting is useless to him, as he could be considered paralyzed. Let's be lenient and say that he has been lucky and knows Still Spell, Silent Spell and has taken the Eschew Materials feat. :)
He still cannot cast spells with expensive material components or focus. Neither can he take any action that physically moves him, as he is still considered melded into stone. Last but not least, he cannot prepare new spells unless someone or something is kind or obedient enough to hold a spellbook in front of him (or in front of his familiar). So the spells he has remembered are those which he prepared more than 10,000 years ago, right before he melded into the rocks. Would he have prepared all these spells as Still/Silent spells? Doubtful.
The best poor Vordakai can hope for is someone coming along that destroys him, so he can reform non-paralyzed near his phylactery. The second best is to dominate a capable minion that turns the pages of his spellbook so he can devise a way to escape in a magical way.
Enter Noleski Surtova. First he's quite afraid of the lich, then he realizes that Vordakai doesn't pose a threat in his current condition. He cannot retrieve the Oculus without destroying the petrified lich. But by doing so he releases the age-old wizard (let's agree that Noleski doesn't know the location of the lich's phylactery). So he strikes a bargain with the creature: Vordakai is to help Noleski with his Oculus, and in return Noleski pledges either to destroy/free the lich, or get his spellbook. Vordakai sends his familiar with Noleski, doing his neat trick through his telepathic link with the creature. Noleski, however, isn't planning on keeping his promise to the elder creature and tries to kill the familiar as soon as the task is complete. The raven escapes, however, returning to the lich.
In the following years, the new regent is kept busy governing the realm, continuously postponing to deal with Vordakai, who remains, after all, locked somewhere far away from the Dragonscale Throne. Probably Noleski has taken the lich's spellbook in order to make sure that Vordakai will not find some gullible creature to help him escape. Vordakai sees the centaurs as nothing but animals, so he doesn't try to 'transport' them to his tomb. Patiently he waits. Over the years some poor souls find and enter the tomb, but none of them are able spellcasters, useful to the lich.
Then Maegar Varn arrives in the Nomen Heights region and begins to build a settlement, so Vordakai begins to plot a devious plan to take revenge on the Surtovas. Through his familiar he spies on the Varnhold citizens until he spots the arrival of the wizard Cephal Lorentus. Some time after that Vordakai uses the Oculus to grab the wizard and the whole Varnhold population to his tomb - the wizard because the lich hopes he knows of some spells that can release him, the rest because they can serve as (undead) minions in the army he envisions. For Vordakai's goals, however, the wizard is next to useless. Probably a Freedom or Disintegrate spell or adamantine weaponry is needed to release/destroy him from the stone(?).
Enter the PCs. Most of VV can be played as written. Minor changes in the tomb concerning Vordakai's appearances. It becomes interesting when the PCs finally encounter the petrified lich: destroying him will only release him, unless they're able to find his phylactery (a possible high-level quest in itself), while leaving him alone is no final solution either. Perhaps the PCs can temporarily ally with Nyrissa to find a means to defeat him, who is herself concerned over the re-awakening of such a powerful foe in her Stolen Lands. Vordakai, for his part, will do anything to convince the PCs to ally with him, possibly against the Surtovas and/or Nyrissa.
In any case, the lich is 'preserved' for a higher-level climax, unless the players are smart enough to deal with him presently.
So, are there any narrative and mechanical loopholes I'm missing?
I always felt that Starfall, and perhaps the Golka clan, were somehow tied in with the vanishing.
Not bad, different but not bad. I don't see any gaping holes. My version is simply that the Rogarvians made a deal with Asmodeus for the service of 2 red dragons. The "deal" allowed Asmodeus to claim the family 200 years later. If my players took too much an interest I'd add the dragon as a villian back into the mix and throw the contract as a hidden item in his hoard.