Archiving in Exchange 2007 and 2013 coexistence

I have Exchange 2007 and 2013 in a coexistence environment. I am ready to start migrating mailboxes. I am using tech48928 as a guide. I have completed all of the tasks including creating a system mailbox for my 2013 server. According to the doc there should be a system mailbox for each server that will be archived. Now I understand that my 2007 vault service account needs to be moved from 2007 to my 2013 server. I believe that my 2007 system mailbox and vault service account mailbox are one in the same. So, if I have to move the vault service account mailbox do I need to create a service a system account mailbox for 2007. I may be totally confused.