I have used the Cisco VPN client for years, and on XP it is pretty much rock solid, on Vista, not so much. You might be able to connect, but unless you set up a constant ping to something, it usually will disconnect, with this error:
Reason 442: Failed to enable Virtual Adapater
Now, most sysadmins will say, update you client - yes, I know, it hasnt mitigated the issue, ever. I have been using it on Vista since it came out, and updated my client every time a new update comes out.
I have used it where the VPN server was a Cisco PIX and a Windows Box allowing the Cisco Client to connect.
Anyone have any fixes that you can do on your client to fix this issue?