Decrypting Vanitygen Protected Key
Buried in the bitcointalk.org forums I found this answer.

Use the keyconv binary file that is included, with the protected/encrypted key as the only  argument:

keyconv [Protected Key Here]


This will spit out the unencrypted private key. It isn't documented anywhere other than this forum post:

https://bitcointalk.org/index.php?topic=25804.msg2107686;topicseen#msg2107686