Hi,
SimpleSpleef v3.1 (https://github.com/Kaosvf/SimpleSpleef-Evolution) + Vault v1.2.17 + CraftConomy v2.3.5 (http://dev.bukkit.org/server-mods/craftconomy/files/8-2-3-5/) when someone do "/spl join" in console appear this: http://pastebin.com/MfWgGj8c
Doesn't happen with (for example) iConomy and I don't understand how to fix it, can you kindly make a pull request to the bugged project?
Thank you.

It says the error is caused by this:

me.greatman.Craftconomy.Craftconomy.format(Craftconomy.java:355)

I don't think this is part of Vault

You should contact the great man :P

as @Mtihc said, this is an issue in CraftConomy.