Enterprise Vault archives messages from the journal mailbox, but it doesn't actually contain functionality to perform the journaling.
Depending on message volume, you might be able to get away without a dedicated Exchange server for journaling. However, I would highly recommend at least a dedicated storage group on its own disk spindles. Normally, EV empties the journal maibox once per minute, but if journal archiving fails or is down for some reason, you don't want to fill up the storage groups containing user mailboxes. Exchange is also disk I/O intensive so you need to be careful about significantly increasing the load by adding journaling to existing disk.
