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In order to test VPN connectivity, you should ping an internal SLAC only server; an example would be www-lanmon.slac.stanford.edu
Why don't you support Mac OSX 10.5 on a G5?
Regardless of getting VPN to work, Apple is currently at Lion (OSX 10.7). PPC/G5 Macintoshes are not supported under 10.6/10.7, so plans to replace such systems should be instigated for security/patching reasons alone.
The Cisco AnyConnect client for PPC is stuck at version 2.5. Although Cisco has been releasing security patches for that version, it is one major release behind the current software for all other platforms, which is at 3.0.
We performed significant testing of the 2.5 client, including on PPC systems, but eventually the the decision to support only 3.0+ client versions was made due to the small number of users who would be affected, and the assumption that Apple will not continue to support PPC systems in the future (we don't know for sure, but are making an educated guess based on Apple's past behavior).
Are you doing your own testing with a Mac? Or a Windows box?
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Stanford university is using different VPN protocols and technologies, so the SLAC VPN client can't be used to connect to Stanford through VPN. But when connected to SLAC VPN you can reach Stanford ressources as if you were in your office at SLAC.
If you are a SLAC employee/user willing to access SLAC ressources you should use SLAC VPN, if you are a Stanford university employee/student you should use the Stanford VPN client provided on http://itservices.stanford.edu/service/vpn.
Problems with VPN client for Mac OSX10.9 Maverick
A subset of people running Maverick is experiencing VPN connections being dropped every minute.
This bug was acknowledged and a special version fixing it is available to Mac users at: V:\SCS\ITDS\Apple\anyconnect-macosx-i386-3.1.04074-k9.dmg (windows share) or at /u/sf/gcx/vpn/anyconnect-macosx-i386-3.1.04074-k9.dmg (AFS).
Only affected people should use it as it is not yet considered production ready. We are still waiting a more stable release to have it automatically pushed by the VPN gateways for everybody. This was expected beginning of December, but will more likely be beginning of next year as we do not want to do changes on VPN around end of year holidays.