Okay, so your master PC has three distinct network cables running from it, right?

I assume that when you unplugged everything, you just unplugged all the cables except for the one that runs from the master PC to the modem.

Unfortunately, that doesn't really take the "Network Connection Sharing" out of the picture.

Would it be feasible to plug the second PC directly into the modem (configuring its network so that it is able to access the internet that way, obviously) and install the VPN client software on it? If so, do that. If it still doesn't work, it's likely that something in your ISP's network is preventing the VPN from working. If it does work from there, we can assume that there's a configuration problem with the "Network Connection Sharing".

Actually, one thing you might check first is this: open your Cisco VPN client, modify the appropriate configuration entry, click on the "Transport" tab, and see if "Enable Transparent Tunneling" is checked. If not, check it and try again, if it is, try changing the protocol to the other one. Basically, try every possible option: transparent-disabled, transparent-udp, and transparent-tcp.