It may surprise you but Chromebooks have VPN functionality built in. However, it is still a beta feature and is deactivated by default. Here are the steps to activate it. Type in about:flags in the address bar and enable "VPN support" from the list of experiments.
Then, you can easily add layer 2 tunneling protocol (l2tp) and other related features easily. Right now, Chromium OS only supports L2TP/IPSec with pre-shared key and user certificate. In my testing, it worked with Windows Server and Cisco, but since it is still an ‘experimental’ feature, it may not work with all of them.
