Just because you have the messages available in multiple formats and locations doesn't change that it's an attribute of the envelope not meant to be rewritten. The functionality is just not built into any tool I know of. Having that functionality would create the ability to screw with discovery (I mean, there could be mitigation with versioning, but that would need other configuration)
While it may not be a read-only part of the envelope(I'm not actually sure), the only tool that MIGHT be able to do what you want is MFCMapi, and I don't think you want to play with that for this job. The chance of getting it wrong would be pretty high I think and it is not a particularly friendly tool. I'm not sure it could be scripted with it either.
My recommendation would be what /u/borismkv said. Making a mailbox for VIP and telling them to use that. Forwarding to VIPs mailbox would be ripe for them to just respond directly instead of responding through his relay mailbox.As for your existing messages, if the current users absolutely cannot see the existing messages, you'll need to do a search and export and just forcibly remove the messages from their mailboxes. It's not clean and not advised by me, but if they don't want VIPs address out there it will need to be removed. I would do a search with his email address as the query with -LogOnly -LogLevel Full and see what kind of results you get.