hello
if the vpn is terminating on an external contivity box, u need not do anything in ur 804 router... you just need to add ur dialer list to trigger the ISDN when ur IPSEC traffic or any other traffic hits the router... i hope u dont have any access-lists on ur router.. you just need to have a static routing on ur 804 towards the dialer....
hope this helps.. all the best
Raj