I have another plugin that calls this method and the NPE is only present when mchat is loaded. Disabling mchat stops the problem and the plugin works as expected.

18:53:13 [SEVERE] java.lang.NullPointerException
18:53:13 [SEVERE] at net.milkbowl.vault.chat.plugins.Chat_mChatSuite.getPlayerPrefix(Chat_mChatSuite.java:106)
18:53:13 [SEVERE] at net.milkbowl.vault.chat.Chat.getPlayerPrefix(Chat.java:70)

yup there's currently an issue in either how mchat is loaded, or mchat itself that is causing the plugin to return null values for it's API.