Information

Full output of /ess version:

[19:31:58 INFO]: Server verze: 1.15.2-R0.1-SNAPSHOT git-Paper-146 (MC: 1.15.2)
[19:31:58 INFO]: EssentialsX verze: 2.17.2.13
[19:31:58 INFO]: LuckPerms verze: 5.0.72
[19:31:58 INFO]: Vault verze: 1.7.2-b107
[19:31:58 INFO]: EssentialsXChat verze: 2.17.2.13
[19:31:58 INFO]: EssentialsXSpawn verze: 2.17.2.13
[19:31:58 INFO]: ChestShop verze: 3.10.2-SNAPSHOT (build 176)

Server log: https://gist.github.com/Iwitrag/85d7a248c836286a37add505bdb22b29

EssentialsX config: https://gist.github.com/Iwitrag/3dd435516f32005017a77876e088c7db

Details

Description
Vanish command has various behaviour based on which command is used to execute it.

Steps to reproduce

1. Have player count in TAB list header
2. Do /vanish
3. Player count decreases by 1
4. Do /vanish
5. Player count returns to normal
6. Do /v
7. Player count unchanged
8. Do /v
9. Player count unchanged

Expected behavior
I would expect that all aliases behave the same

Unable to replicate this on the following version:

[12:37:18 INFO]: Server version: 1.15.2-R0.1-SNAPSHOT git-Paper-79 (MC: 1.15.2)
[12:37:18 INFO]: EssentialsX version: 2.17.2.71
[12:37:18 INFO]: LuckPerms version: 5.0.72
[12:37:18 INFO]: Vault version: 1.7.2-b107

In addition, looking at the code, there is no reference to changing behavior based on which alias was typed in. /vanish is identical to /v. For that reason, I'm going to go ahead and close this issue already now. Most likely you miscounted, or perhaps /v is being registered to another plugin. If this is the case, /essentials:v will work as you expect it to. You could fix the alias in your commands.yml file to ensure that it is registered to Essentials.