Browse to http://192.168.0.1/ (on a computer connected to the LAN) to access the cable modem admin tools.
The default username is admin and password motorola. (If you have changed and then forgotten the password, press reset for 30 seconds—the reset "button" is the tiny pin hole at the top of the back plate—after which the default credentials should work again.)
Once logged in, go to Gateway -> LAN -> DHCP Server Config. You should see something like this:
Now, set LAN IP Address to 10.0.0.1, and Starting IP Address to 10.0.0.2
Click Apply, and that's it! For the change to take effect, you still need to renew DHCP lease on each client computer so that they get a 10.0.0.X IP address.
Remember that after this change, the cable modem admin page will be available at http://10.0.0.1/
In my case, this indeed helped with the VPN problem: now when VPN connection is on, I can access both the private network and public internet.