First of all, what is the MTA that you use in your organization? if you have Brightmail it will very much assist in the resolution of your issue.
The console in which you approve the sending of the mails is a Brightmail console not Vontu.
I have not tried it, but allowing managers access to the console could resolve the problem.
I think that if you have any programmers around they can write a script that will be inserted to the mail sent to the manager, and when he clicks approve the script will open a session to the console and auto-approve the mail, it wont be easy but Symantec DLP already does 50% of the job by resolving the manager name and mail with the LDAP plugin.
Regards,
Naor Penso