To clarify: What is happening is that the ip address leaving the machine, if not on the same network, always gets translated to the ip address of the gateway, so that traffic never gets beyond that point.
When I plug in my windows laptop to that network port and use the ip of the unix machine, everything works correctly, the ip addresses don't get changed. My windows machine can ping remote addresses and get beyond the gateway. The same is true for a Solaris machine on that same switch. Traffic can be detected going from the Solaris machine to machines on other networks - for example, I can successfully ping Yahoo, Cisco, Google..etc. and I can see those destination addresses in the sniffer.
On the HPUX machine however, if I ping Yahoo, Cisco, Google or another network on our VPN link, the destination coming from that box always shows as the gateway interface.