Hi Sandrine,
The VSA does not have to have a mailbox, so you can delete the mailbox. If you want to be sure, move the mailbox to another server. It is (afterall) nothing more than a usermailbox. The new systemmailbox you have created is defined to be used in the task, so this VSA mailbox will just sit there, and gather mails sent to 'all' ;)
Regards
Regards. Gertjan