java.lang.NoClassDefFoundError: net/milkbowl/vault/item/Items at org.gestern.gringotts.Configuration.unitName(Configuration.java:112) ~[?:?] at org.gestern.gringotts.Configuration.parseCurrency(Configuration.java:242) ~[?:?] at org.gestern.gringotts.Configuration.readConfig(Configuration.java:164) ~[?:?] at org.gestern.gringotts.Gringotts.reloadConfig(Gringotts.java:224) ~[?:?] at org.gestern.gringotts.Gringotts.onEnable(Gringotts.java:99) ~[?:?] at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:264) ~[patched_1.12.2.jar:git-Paper-"d934dcc1"] at org.bukkit.plugin.java.JavaPluginLoader.enablePlugin(JavaPluginLoader.java:316) [patched_1.12.2.jar:git-Paper-"d934dcc1"] at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:404) [patched_1.12.2.jar:git-Paper-"d934dcc1"]
According to your plugin.yml the vault plugin isn't required, but if Vault isn't installed on the server this error pops up.

Install Vault, it is required. docs probably need updating

Technically a bug because it shouldn't be required, but someone would want to fix it

Maybe shade/shadow net.milkbowl.vault.item.* into the jar?

Vault no longer handles items as per #182