There are Vampire Coast rules in the homebrew arcane journal by ASF (
link), though I haven't really dived into them or seen them in play. There's also this homebrew Vampire Coast pdf by Daruos_XIII (
link) which I also can't speak to the rules quality but the formatting and presentation at least are very nice. Outside of that, as far as I know vampire coast would just be a modeling/conversion theme for generic vampire counts, which are highly competitive in Old World official rules due to our legacy pdf being basically the most powerful and most fully fleshed out of the legacy armies, and undead rules generally (everything unbreakable, lots of healing) being very strong in the current Old World ruleset and meta.
That said, due to the overall lack of new rules updates from Games Workshop there have been some major community homebrew projects - including ASF's homebrew arcane journals expanding the legacy factions but more notably the Square Based crew's 'Renegade' legacy faction reworks - which also touch up vampire counts with a couple nice quality of life changes, but that mostly leaves our rules alone since we don't need the help. There's also various comp systems including one from Square Based. Many of these projects are intended to level the otherwise rather lopsided faction balance of the competitive scene. Which is a goal I support, but again given that vampire counts are currently near the top of that scene such projects inevitably reduce our faction's strength, at least relative to other factions.
So I guess to answer your question, if by 'viable' you mean 'competitively strong' AND you're using the normal Vampire Count rules in a by the books GW rule environment than the answer is "yes they're very viable"
If you're still using normal vampire count rules but in a rules context that uses homebrew rules to patch up the other legacy factions or apply a comp to try and level the competitive space, then the answer is "we're still viable, but much less dominant".
And if you're talking about ASF's vampire coast variant rules specifically, then the answer is "I have no idea."
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If you aren't talking about competitive viability at all, but instead more of a 'does a rules framework and model support exist to run this faction at all', then the answer is still 'yes, it's a viable army', but exactly how viable depends on how open your local old world scene is to homebrew and non-GW models
Obviously if your local scene allows ASF's homebrew journal then you have a usable rule set. If not, then you're stuck with the vampire counts rules. People have been using the generic vamp counts rules to represent vampirates since the oldhammer days, but there are some limitations. Some of the specific units you might know from the Total Warhammer games don't have a good translation, including the zombie ogres and the monster stuff. More notably the elite vampire infantry unit doesn't really exist in
VC, only a cavalry version that feels less fitting for a pirate themed army. Also anything ranged of course - though a tomb kings allied detachment can help out there, with skeletal archers to represent undead pirates with small firearms and the screaming skull catapult to represent larger artillery like a big cannon or mortar. Not a perfect 1 for 1, but good enough rules base for a conversion project.
In terms of something that doesn't necessarily exist in the total warhammer faction but that would make for a cool centerpiece/thematically fitting kitbash project - the big floating chariot units in the vamp counts army are pretty good, particularly the mortis engine, and that unit's rules are just begging for a big flying ghost ship conversion project.
Otherwise, you've got rules for various undead heroes, and you've got rules for skeletons and zombies and ghosts. That's pretty much all you need for a vampire coast army. You'll be weaker for not fielding cavalry, but again vampire counts by default are a bit op in old world, so a bit of thematic restraint won't hurt much.
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Model wise, people have been converting vampire coast armies from generic vampire counts for ages. You can certainly still do that, though admittedly it is harder to do with the current Soulblight Gravelords
aos range since those models are significantly more specific and less modular than the older warhammer fantasy vamp counts range. So yeah, it can still be done, and that's the way you have to do it if you want to play in GW's own hobby stores & official events. It'll be a project, and you'll need to get comfortable with green stuff sculpting, which is a nice hobby skill to have anyway. Definitely lean into the zombies - GW's zombies have some weird roots growing through them and many are stooped in awkward ways, staked to random tombstones or whatever, but you can leave the tombstones off, trim the roots, and reasonably work with what's left over, greenstuffing over holes, adding some eyepatches, hook hands, peg legs, bandannas, & tricorners. The current skeletons unfortunately will be a lot harder. all of GW's current skeleton offerings are heavily armored, with a distinctive and not-very-piratey armor design. Making those into pirates will probably be more difficult than they're worth, though clipping off their furred hats and sticking a greenstuff tricorner on their head could go a long way. That said, you're probably better off converting skeletons from the currently available old world tomb kings models. They're goofy and poorly proportioned, but a lot easier to convert from since they start off naked. If you can find any of the old 7th warhammer fantasy era vamp counts skeletons though I'd absolutely buy those up. They have an awkward forward lean, but are otherwise very nice to work with. GW ghouls are very stooped, but there's a more elite 'crypt guard' unit in the current range that stands a bit more upright and so might be a bit easier to work with. The fell bats are good models that can pretty much be used as is. For heroes, especially vampires and necromancers, I'd look to the underworlds range since those models generally stand on smaller bases, and convert from there.
If you're playing in an independent hobby space you can get away with non-gw models, and in that case there's tons of options, as undead pirates is a rather common trope. Lots of third party models available if you look, lots of 3d print files if you have access to a resin printer or know somebody who does. fdm printed minis is becoming more of a thing these days, but is still less viable for undead models which tend to be more fragile and spindly. Be warned before buying a resin printer though - printer resin is highly toxic and the fumes are super bad for you. It's going to need basically it's own room, one that is both shielded from sunlight and heavily vented to the outdoors, which can be an awkward combination. If you have pets or kids it's a bad idea, and if you live in an apartment or condo then you can basically forget about it. If you need help finding specific models or stls for third party vampirates I could try to help, but honestly they're common enough that just searching around should find you stuff.
