Most likely the outlook profile on the computer that connected thru the vpn caused the issue.
I would suggest using exmerge to save the users email. delete the mailbox. then recreate it in active directory for your new exchange server. then use exmerge to restore the email.. then be sure to correctly redo his outlook profile under his login, if you can create his outlook profile for him on the computer he uses for via vpn. or have him use outlook web access instead of outlook from his vpn connected computer.