A very small mod that allows to increase overworld border by completing vaults
Description
Vault Hunters World Border Extend Mod
This very simple mod allows extending the overworld border on a player exiting the vault.
The config file allows for the definition of values, and the lang file allows for changing messages that Vault Gods are sending.
This mod requires an extra mod that enables per-world dimension. Several mods work together well, but this may be the best option as it is server-side only:
https://www.curseforge.com/minecraft/mc-mods/multi-world-borders-unofficial
However, you can try any other mod too.