A pix firewall should allow a dynamic client by using the peer Ip as 0.0.0.0
This will work on most vpn firewalls.
Have you tried it?
http://www.cisco.com/en/US/products/sw/secursw/ps2308/products_configuration_example09186a0080094680.shtmlthe link shows the idea between to pix units and so it shoud still work fine with a draytek just so long as the unit with the dynamic address dials the static address unit.
If you have tried it, then I will keep quiet and stop harrasing you.