How to configure client OS X VPN (L2TP) connection so you do not loose access to intranet? I am looking for a solution for configuring the VPN on OS X so you do not loose access to internal network. 

In fact I want to use the VPN only to enable access to different external services that are cut by the firewall. 

One example would be to enable TeamViewer which, is cut by the corporate firewall. VPN works, but when the VPN is enabled, the access to internal network is lost, and I don't want this.
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"