Enabling PPTP passthrough on ScreenOS

Enable vip multi-port command; this allows a VIP service to listen to multiple ports simultaneously
After entering set vip multi-port you'll be promptd with a warning that you must reboot.

set vip multi-port
save

reset

Define the service as usual

set service CustomPPTP group "other" 47 src 2048-2048 dst 2048-2048      

Append the TCP source ports to the service we just defined

set service CustomPPTP + tcp src 0-65535 dst 1723-1723

The source port for TCP 1723 must be 0-65535 to allow for any source port

Set up your VIP   

set int eth0/0 vip interface-ip 2048 CustomPPTP 192.168.1.2

And define your policy

set policy from untrust to trust any vip(ethernet0/0) CustomPPTP permit log