There are many glaring issues currently with VC in campaign, let me give you an overview:
1: Settlement buildings.
Owning a minor settlement currently offers no benefits apart from a total of 300 gold income maxed out - after investing about 15k gold. That's horrible.
The money building has atrocious return for investment even for major settlements, increasing in steps of 50 gold at a cost of up to 4000!
Effects you would expect, such as hero capacity increases, are inexplicably absent. Long story short, if a settlement doesn't have a port it's a complete waste of time and money, and even if it does it feels lackluster.
You NEED to add some benefits to these buildings, for example hero capacity, increases to pirate cove income, recruit rank/slots, ambush spotting (lighthouse), a combat ability in the province (say, one charge of turning an enemy unit >20% hp into a unit of Syreens for the Night Pyres), diplomacy/allied recruitment/trade bonuses for the tavern.
2: Settlement battles and VC units.
The early available VC units are horrible in uneven terrain, especially settlements. Your melee units are guaranteed to lose against pretty much anything, and your gun units hardly ever get to shoot. Really the only option you have is to blob enemy infantry up with your lord/hero, then damage them with bombers/mortars. Dogs get stuck and start crumbling even when attacking archers from behind, and anything not engaged in melee will dodge mortars (and Queen Bess) with about 80% success rate (even shooting archers dodge about 50% of mortar shells in between).
I suggest making Animated Hulks tier 1 (and available through raise dead), giving them the wallbreaker trait, and reducing their upkeep. This will set them apart from Mournguls a bit more, and make settlement battles a little less painful. Further suggestions would be to add polearm deckhands to raise dead, and increase handgunner range to 160. Because VC also mightily struggles against cavalry and archers (peasant bowmen completely and utterly destroy any zombie unit at 160 range, in any terrain and from behind other units, and they outrun them too).
3: Pirate Coves.
The hero action to install a pirate cove is unsatisfactory. While the cost to payoff ratio in itself is just barely ok, the 15 turn cooldown per character has to go. That's 3750 gold upkeep per hero in between. Plus ~2000 for installing, plus 2500 for the building - for 200 gold per turn. I suggest no cooldown at all, as the cost is already high enough.
Furthermore, the diplomatic penalty for giving a cove to an allied faction needs to go. In fact, the coves should provide minor bonuses to allies as well (similar to outposts), and slightly improve relations if anything.
In addition, if you gift/trade away/lose control of a coastal settlement, it should come with a preinstalled pirate cove (could be a tech, or the effect of a regular building). As it stands, you lose a pirate cove if you take a settlement - forcing you to leave enemies alive you'd really rather not.
4: Lack of new mechanics/diplomacy options.
VC cannot vassalize or confederate. They have no bonuses to diplomacy (coercive or otherwise) or allied recruitment at all. They have no access to spellpower at all (aren't vampires supposed to be powerful casters?).
5: Seafaring - dark elves do it better. And ship growth.
Black Arcs can attack coastal settlements directly from the sea - VC special lords with their own ship need to be able to do the same. Also, dark elves have a sealane in range of their main settlements (Malekith and Malus), connecting to further sealanes. Please add an equivalent (or more) for the vampire coast.
With the recent changes, VC ship growth has been nerfed severely. Now you need up to 32 growth to reach tier 5 units, and access to the first caster hero has been delayed as well. This feels terrible, especially because you need higher tier units to even the odds after being so much weaker than all the factions with archers and cavalry and effective melee from the start.
Curse of Undeath seems too weak. It heals for 0.35% per cast - that is total, not per second. A bit ridiculous, no? Nurgles passive is 3x as strong (lore of life too, I think).
Mourngul Haunters miss their charge attacks against single entities, they turn very slowly and get staggered a lot, and cannot push past even peasants (mass issue might be related to campaign difficulty). Very underwhelming performance, can you make their hitbox smaller, make them more agile and maybe give them a bit more mass?
Please add a Syreen hero that has the passive of increasing movement range (see Banshee).
If a Bloated Corpse starts the battle at low HP, it will immediately explode (and possibly break the game if the enemy tries to shoot it). Similar with Necrofex, they should need to be hit for their passive to trigger.
Syreens were tanks, not damage dealers. The buff to AP does little for them, yet they are less tanky and thus worse than before (they get healed less is one of the issues). Similarly, the Shadewraith Gunners just got a lot worse.
The Black Spot is a melee/ranged hybrid, but lacks the HP to fight in melee. Maybe buff to equal deckhands?
The Night Terrors still have rampage... that's terrible.
You could give all crabs reverse shields (45% missile block from behind). Would be cool, I think. Also, no-gun-prometheans could use a buff or something, they seem pointless (same building has Mournguls).
Deck Droppers are so weak in campaign, I'd never have more than one in an army - and then only to chase of routing units. Yet you nerfed their red line buff from 20% to 8%? Can you make them at least passable in melee or something (flying units in general are absolutely horrible at killing routing units, only a few models will attack).
Deck Gunners should be tier 3 instead of 4. They die like instantly if anything attacks them, have no shields, crumble, are useless in siege battles... at tier 4 I'd rather get crabs.
Bloated Corpses have "causes terror" on their card... but they don't seem to cause terror? Bloated Corpses should alsom break a wall segment upon explosion (and be able to attack walls).
The landmark in the Black Pyramid of Nagash was missing when I played as Luther Harkon.
The "determined" trait is useless for VC, as all heroes are immune to psychology by default, and +3 leadership for the hero only is pretty much nothing. Knowledgeable is also horrible. You removed a trait that had +10% research and -10% build cost (if I recall correctly), may we get that instead?
Disbanding a lord will disable his fleet office effect - you have to manually remove and reassign him. A minor bug.
Can we have a better dog version, one that doesn't crumble when attacking archers, and maybe with poison?
I hope you find this useful and will take it into account, as I really like the VC, but I'd currently rather play something else...