Eraser is correct with regards the 2 configuration setups. The control to change this is buried in the config panels of the Network Manager Open VPN setup (There is a checkbox with an option like "Use this connection for all routing" which, when unticked, will allow you to connect to the Internet through your normal connection and only use the VPN for IPs configured on the VPN network).
Alternatively, depending on the interface (not sure which type you're using), there should be a "Only use VPN for these addresses" checkbox with a text input to specify an IP range.