Hi,
If Router/Firewall/VPN is the same box,
the outside ip would be the one assigned to your dialer interface. You can have the class C nettwork assigned on the LAN int of the 2821.
If you want a portion of the class C nettwork on the outside, and your provider agree you can use ip unnumbered fa0/0 on your dialer interface.
HTH
Regards,
Bjornarsb