To read more about it: MassiveCraft/Factions#366

Here is the startup, the command that causes the stacktrace and the stacktrace itself:
http://pastebin.com/RXVqQsur

I'm using the iconomy 5 version that is updated or maintained by the towny guys, downloadable on the towny page.

The only way for this to happen is if iConomy didn't actually load, or you modified the class names of the iCo5 version you're using, are you using a custom version of iCo5?

unfortunately, it looks like something has re-written the iConomy plugin variable in the Vault connector. I'm baffled at what's going on here as this connector is over a year old and no changes to startup or errors. It's highly unlikely this has anything to do with Vault and more than likely some odd plugin interaction.

ChatManager.jar iConomy.jar Modifyworld.jar Questioner.jar Towny.jar
Factions.jar mcore.jar PermissionsEx.jar TownyChat.jar Vault.jar

Those are all plugins I'm using. All are straight from dev.bukkit except for towny and iconomy5, that is from http://palmergames.com/downloads/

I do not experience this issue with Vault + iConomy and any other plugin that uses economies, I do not use Factions, and unfortunately I will not be taking any time to track down what is going on with it.

A) iConomy is an outdated economy no longer receiving functionality improvements or updates.
B) the iConomy 5 plugin and Vault connector has not changed in more than half a year.

There's no feasible explanation as to why the error would be due to Vault.