getPlayerPrefix is probably attempting to pull a world-specific prefix. Can you pastebin the full NPE and link here?

that's an NPE in your own plugin, not Vault.

@Tazzernator - should it? what happenes if you hook PEX directly and PermissionsEx.getPermissionHandler().getUser(playerName).getPrefix() ? cause that's all the Vault method is doing in this case. I'm guessing you're actually trying to hook group data not user data though. I had issues similar to this when I hooked PEX directly (before Vault)

You'll probably need to check if the player has a prefix or if their group has a prefix in this case.

@Tazzernator - were you able to resolve this or figure out what the difference was?