While having the vanish mod on my VaultHunters server my displayed players never go over 12.

Hello Mareron, glad to hear that changing the config value fixes the issue! Just out of curiosity, was your player count stuck at 12 as well, or was it a different number? And were you also using the Vault Hunters mod pack or a different set of mods?

It was also stuck at 12, yes.
We are using a different set of mods.
Pixelmon and Oh the Biomes you'll go and a whole bunch of utility/content plugins

Fixed by be9fb69, new version will be released in a bit

Hello, I assume all of the 19 players online are not vanished? Also, what version of Forge and the Vanishmod are you using on your server?

Correct, no one was vanished
Vanishmod-1.18.2-1.1.10
Forge-1.18.2-40.2.1
although I had the same issue a while ago with a previous forge version.

Does the modlist your server is using contain any extra mods/misses any mods compared to the modlist of the "Vault Hunter 3rd Edition" modpack on CurseForge, other than that the Vanishmod is installed?

Extra mods:
alternate-current-mc1.18-1.2.1
block_log-1.0.0+1.18.2
canary-mc1.18.2-0.1.8
Chunky-1.2.164
dcintegration-forge-2.5.0-1.18.2
flan-1.18.2-1.8.2-forge
ftb-essentials-1802.1.7-build.42
itemblacklist-1.18.2-1.2.3
lazydfu-0.1.2
logprot-1.18.2-1.6
LuckPerms-Forge-5.4.26
ModListObserver-1.18.2-1.0.1
pluto-mc1.18.2-0.0.6
quickspawns-1.18.2-1.0.3-forge
saturn-mc1.16.5-0.0.3
starlight-1.0.2+forge.546ae87
starterkit_1.18.2-3.9
Tempban-1.18-1.0.1

Missing:
FTBBackups

I'm afraid to say that I currently do not have the computing capacity to run a Vault Hunters server + client(s) to at least try to reproduce this issue, so this might take a while for me to properly resolve. I might be getting a better PC in the future, in the meantime it would be incredibly helpful if you

1. could test whether this issue also arises if the Vanishmod server config option hidePlayersFromPlayerLists is set to false
2. could test whether this issue arises with only Vanishmod installed, and if not, do a binary search (= remove half of your mods, see if the issue is present, if yes -> test with half of current mods, if not -> test with unused mods) to find out which mod(s) cause this incompatibility (if it even is one).

Thanks a lot in advance, and sorry that this issue might not get resolved as quickly as expected.

We've experienced the same issue but did find a fix.
In /world/serverconfig/ you'll find a vanishmod-server.toml file

In there you can set
#Should vanished players be hidden from player lists such as the /list command and the list in the Multiplayer screen?
hidePlayersFromPlayerLists = false

that fixed the issue for us