When I compile my plugin using Maven, I get this message in the log:

[WARNING] The POM for net.milkbowl.vault:Vault:jar:1.2.27 is invalid, transitive dependencies (if any) will not be available, enable debug logging for more details.

When I start my plugin, my Economy object returns null. I think this is linked to the warning message above.

This is normal.
As for the economy - do you have an economy installed? if so, which one?

The warning message during build time has nothing to do with your server not finding economies at runtime. Your economy object most likely returns null because you have not installed an economy plugin.

We don't support custom builds of Vault, you should be downloading it from http://dev.bukkit.org/bukkit-plugins/vault/

If you're trying to build a plugin that utilized Vault you are using the wrong pom. You should be using http://github.com/Milkbowl/VaultAPI to build against.

Also, 1.2.27 is outdated you'll need to be using 1.4 to get support, thanks.

Hope this helps!

I am using 1.4, it says 1.2.27 for some reason. And yes, I'm an idiot and forgot to install an economy plugin on my debug server. Thanks for all the help, sorry for wasting your time :(