I have the latest release of towny, as suggested on the DL page.

I get this error:

2012-07-15 09:35:29 [SEVERE] Error occurred while enabling Vault v1.2.15-b172 (Is it up to date?)
java.lang.NoClassDefFoundError: com/palmergames/bukkit/towny/NotRegisteredException
at net.milkbowl.vault.Vault.loadChat(Vault.java:214)
at net.milkbowl.vault.Vault.onEnable(Vault.java:109)
at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:215)
at org.bukkit.plugin.java.JavaPluginLoader.enablePlugin(JavaPluginLoader.java:337)
at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:381)
at org.bukkit.craftbukkit.CraftServer.loadPlugin(CraftServer.java:256)
at org.bukkit.craftbukkit.CraftServer.enablePlugins(CraftServer.java:238)
at org.bukkit.craftbukkit.CraftServer.(CraftServer.java:190)
at net.minecraft.server.ServerConfigurationManager.(ServerConfigurationManager.java:53)
at net.minecraft.server.MinecraftServer.init(MinecraftServer.java:166)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:432)
at net.minecraft.server.ThreadServerApplication.run(SourceFile:492)
Caused by: java.lang.ClassNotFoundException: com.palmergames.bukkit.towny.NotRegisteredException
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:41)
at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:29)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 12 more

you don't have an up-to-date version of Vault.