It sounds like the settings in the computer are set up so that it is requiring the VPN for all connections...
You might be able to set them up as 'Work' and 'Home' and differentiate the settings for the two, so that work connects with the VPN and home connects using the modems own routing functionality.
You might be able to set them up as 'Work' and 'Home' and differentiate the settings for the two, so that work connects with the VPN and home connects using the modems own routing functionality.