Hi Ray,
I am not sure of the existing configuration on the firewall, but you might wanna look into setting the ASA in transparent mode. Transparent mode ASA running version 8.0.2 and above support NAT and PAT.
Here is some documentation on Transparent mode:
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_configuration_example09186a008089f467.shtml#backinfo
This way you could connect the ISP router to the ASA, and the ASA to the VPN router, with all of them having the same subnet ip address. If the ASA is currently in production, in routed mode, then you will have to schedule a downtime to set this all up.
The easier and better way, would be to have a hub or a switch, to which all 3 devices connect.
Hope this helps.
-Shrikant
P.S.: Please mark this question as answered if it has been resolved. Do rate helpful posts. Thanks.