If you haven't heard yet, the devs at Minetown have brought us an awesome, custom, plugin that allows the purchase of vaults with Minetown Credits.
You can check out the article (vault pictures included!) I made at: http://mc.minetown.net/wiki/index.php/Automatic_Vaults
You can check out the article (vault pictures included!) I made at: http://mc.minetown.net/wiki/index.php/Automatic_Vaults