you want to route pptp traffic to a firewall/vpn-device, from there through a tunnel, out of the firewal/vpn-device and then to a pptp server ??? afaik it should work. pptp uses gre packets (ip protocol 47), so it should work over ipsec because it is an ip protocol. but remember that gre packets do not have ports like tcp/udp, so it is difficult to NAT (just like ipsec).
if you have success or you meant something else, let us konw ...
