How to configure client OS X VPN (L2TP) connection so you do not loose access to intranet?
You will need to configure the Routes of the VPN connection. This is a bit advanced and needs some fiddling with the connections.

Theres a guide here
http://www.constrainedrandom.com/2011/mac-os-vpn-routing/

You can also lookup other posts for "Custom VPN Routing"