If this happened in my game (my lot are halfway through TOD anyway) then I wouldn't have Vanthus return to Scuttlecove at all. To me, Vanthus hasn't got much in the way of loyalty to the Crimson Fleet, he's just after power, and the Fleet is just a means to an end for him.
If the battle is lost, he'd have to flee, and as he knows Demo has allies in the centre of the island, fleeing there would seem ideal.
If you can make it look like he's fled below the island, and have the PCs follow down there then up above, you can run TLD and CoBI pretty much as normal... however, it does mean that you have four adventures being one massive 'chase the bad guy, rescue the girl' quest...
Before you do anything, read Tides of Dread. Perhaps you would want to fast-forward to this adventure, since events look a lot like what happened in your campaign.
My reading of it is, at this point, Vanthus doesn't necessarily appreciate who's been pulling the strings on his ascent to power. At the moment, this is more about him wanting his share in the Vanderboren fortune, and some brotherly love for his sister wanting her to share in his glory - or at least not be in his way. His association with the Crimson Fleet and his rise through the ranks have been in part orchestrated by those wanting to use him, but I don't see Vanthus knowing the full details about Demogorgon's involvement until what happens to him after the Tides of Dread.
(read ahead for the intro to Serpents of Scuttlecove and later for more background on Vanthus, and how he was 'chosen' for his path early on without him fully knowing the details)
So I might play this that he uses Lavinia as a hostage, and then works out a way to kill the witnesses in Farshore, so he can claim it as his own, with no one to dispute it. A protracted siege, guerilla tactics etc. Vanthus has charms and dominates to sow confusion amongst those who remain in the settlement. And if he's ever cornered, he can threaten to neevr tell the party where they'll find Lavinia...
Luna - I think you mean Serpents of Scuttlecove, not ToD in your post - but yes, that's another way to approach this
Yes, indeed. I meant Serpents of Scuttlecove.
The problem is that when he uses Lavinia as a hostage now, and kidnaps her again in SOS, you are essentially repeating this plot. Not only is this repetitive, but you also run the risk that the PCs take precautions in order to prevent a second kidnapping and Vanthus cannot kidnap her for SOS.
In my experience, one of my players already found that the Vanthus plot was dragged out too long when they discovered in SOS that he had fled into the Abyss with Lavinia. Luckily Into the Maw is one hell (or one Abyss) of an adventure, so he forgot about his comment when the group got in the middle of the intrigue in that adventure.
But this is seriously something to consider. (And of course I do not know your group. Perhaps my comment does not apply to them at all.)