If you want to have a VPOP3 server and Exchange Server using the same email domain, then you need to ensure that Exchange does not think it is 'authoritative' for the domain in question.
See http://support.microsoft.com/Default.aspx?kbid=321721 for details of how to do this in Exchange 2000 and 2003 (a similar process may be used in later versions of Exchange - contact your Microsoft Support technician if you need more help).
In VPOP3 you would usually use LAN Forwarding to forward the messages for the relevant users to Exchange. See this article for more details.