You can not create VPN on a stick style configuration with PIX running 6.3. PIX 6.X does not support one arm routing.
BUT, if you are too desparate then there is a workaround of creating two VLAN interfaces out of your only physical interface and using the two vlan interfaces as you ingress and egress interfaces. The router in front of the PIX should support the sub interfaces from fastethernets or ethernets.
e0.1 VLAN A ------------------------- fa0/0.1 fa0/1--------------------internet
pix--e0--+ + router +
e0.2 VLAN B ------------------------- fa0/0.2 fa0/2------------- Private LAN
VLAN A outside and VLAN B Inside.
Look for configurations which tell you how to create VLANs on the PIX running 6.3