We do not know if the old vamp counts range will return as an old world faction. There's a lot if speculation, but nothing concrete. Several models are still stuck in AoS, including zombie dragon/terrirgheist, mortis engines/coven thrones, crypt horrors/vargheists, crypt ghouls, hexwraiths (and by extension black knights, even though barrow knights exist to replace them). But there's enough left to make a whole faction's worth even without them, especially if you consider the possibility of bringing back even older sculpts of ghouls, black knights, zombie dragon, and so on.
HOWEVER, even if they could hypothetically release a fully separate old world vamp counts range, we don't know if corporate would let them. The point of keeping Old World and AoS as separate ranges seems to be in order to track sales of the various games separately for the purpose of making financial decisions / making the teams compete with each other for corporate resources. Even if there were technically separate model ranges, some Old World players would still buy and use AoS models because they look cooler, while some AoS players would still use Old World models because they cost less, and this would blur the lines and muddy the sales figures. Corporate would have a harder time determining which game should get what resources, and teams might start cooperating and coordinating to promote each others' sales instead of competing and undermining each other, which in turn would promote horizontal loyalties among labour and thus subvert vertical subordinance to capital. Whether a country or a corporation, a divided population is easier to control.
Specialist Team were allowed to do the dual ranges thing for Warriors of Chaos, but chaos is kind of special. You could have an old world game without vamp counts, but you couldn't have one without chaos warriors, they and empire are load bearing factions. So the fact that this was allowed for Chaos doesn't mean it will be allowed for Vamp Counts (or dark elves, ogres, lizardmen, skaven, etc).
Old World wasn't allowed to keep Chaos Dwarves because they were slated for AoS release, even though the AoS range is going to be entirely new models with no old range holdovers at all. This example is the main thing I would point to in order to say NO, I don't think we're going to get a straight forward port of the old Vamp Counts range to Old World, even after it's all fully replaced in AoS. At least, not any time soon / under current management.
However, I again don't think that's the end of the story. Corporate leadership and its philosophy and dictates change over time. Making departments feud with each other and preventing cross-promotion and spillover sales of your products is obviously leaving money on the table, and eventually someone will be in charge who wants to start picking that money up again. This might be three years down the line, or five years, or ten, but one day it'll happen.
And even in the mean time, the Vampire Coast is ~right over there~, with a really cool aesthetic and narrative concept, and proven appeal via the same Total Warhammer game that is at least in part responsible for the Old World in general and Cathay in particular existing at all. That's something that Specialist Team might be able to sell as a potentially popular entirely new faction that wouldn't unduly cross over with AoS's Gravelords, even though the army would still have zombies, and skeletons, and wights, and vampires, and necromancers, and ghosts. Enough overlap that veteran vampire counts players would likely be able to put together viable armies using their old collections and run the rules counts-as style, & might even see some of the classic range re-released. It wouldn't be a perfect substitute, cavalry and zombie dragons in particular would likely be missing, so old blood dragon players in particular would be left high and dry. But still, it wouldn't be nothing.
And even if considered as their own thing and not as a back door substitute for sneaking Vampire Counts back into the game, Vampirates would still be a cool faction with a lot of potential. Harkon, Noctilus, & such are interesting and somewhat underexplored characters in Old World lore, and the faction could tie in with Sartossa and Tilea which are also commonly pointed to as potential areas for Old World to expand on. Vampirates could also tie in with Cathay via the Jade Blooded vampire lineage mentioned in old lore and the Dead Flag npc faction off the Cathayan coast in Total Warhammer. Warhammer Devs love to pull from pop culture, and we're still not far removed from the global phenomenon that was the Pirates of the Carribean franchise, which was itself full of undeadly goodness and richly overcomplicated lore and fantasy politics that feel almost tailor made to fit perfectly into the Warhammer Fantasy world, with rival pirate fleets feuding and competing with each other but still loosly bound together by a sort of code meant to help them survive in the face of 'legitimate' maritime trade companies backed up by the naval force of imperial powers, whether elvan or cathayan or old world Empire or even Nehekharan. And in the Old World region specifically vampirate fleets would have been bolstered in recent decades by vampires and necromancers fleeing the collapse of Sylvania following the end of the Von Carstein wars. Harkon is already well established on the Vampire Coast at this point in the timeline, mostly active in New World waters but with a fleet already large and powerful enough to start exerting its influence in the Old Worlds seas. And the devs have already proven willing to tweak the canon timeline - it wouldn't be that huge leap to relocate Noctilis to the current era as well. That already gives you at least four proper vamprirate bloodlines/fleets to build narratives around, between the Dread Fleet, the Vampire Coast, Sartossa, and Cathay.
Again, I don't expect anything to come of this any time soon. The rumor buzz at the moment is more around Estalia / Dogs of War or Kislev as the potential next major old world developments. Or alternatively a proper second edition with a cleaned up ruleset and possibly partial range overhauls for some of the existing official factions (Nagash knows I wouldn't complain about new skeleton infantry and horses for the tomb kings). If a second edition does happen, it's anyone's guess whether the pdf legacy factions would be updated to stay playable in any official capacity. So in the /near/ term, things are much more likely to get worse for Vampire Counts playability in Old World rather than better, at least in terms of official playability. There would certainly be community efforts to keep the legacy factions playable, particularly from the Honest Wargamer crew, but I can't say whether any given local scene would be willing to play with such unofficial content.
Long term, though, I am fully convinced that Vampires will officially return to the Old World in some form or another. Whether that's good enough to you to start rebasing old models in the mean time, let alone buying new ones- whether AoS or 3rd party or what have you - is another matter entirely. Personally, if you have any interest in Tomb Kings at all, I would recommend starting there instead.
I've been putting together both Tomb Kings and Vampire Counts for old world - rebasing old fantasy models back to squares after already rebasing them to circles, and as more old models get updated in AoS more of my collection will go back to old world to live on there, which feels like a sort of homecoming. It's nice, actually. Having an 'official' army on the side has made me feel less nervous and insecure about the amount of time I've put into the 'unofficial' legacy army. While I'm sure they'll be back in some sort of official form eventually, and the people I've played against have been happy to allow community/homebrew content, at least I have an alternative option if I ever play against someone who isn't open to that sort of thing.
