Yosemite VPN breaks internet connection over Wi-fi
I have the exactly same issue on both Wi-fi and Ethernet connections. It started after upgrading from Maverick to Yosemite and all my VPN connection were behaving the same.

I got in touch with Apple support regarding this problem and their procedure was as follows:


Create a new network location (which gives us fresh network settings to work with)
Create the VPN connection in this new location
Test


If the problem persists (my situation) then your old settings are not the problem.


Create a new user (administrator rights)
Log in with this new user
Test again


If your problem persists (my situation) then your user settings are not the problem.

At this stage they said the problem should be the VPN providers, but in my situation nothing was changed from their end and I am using their services for more than a year now.

Continuing my research I suspected the problem should be DNS related. I can ping external addresses by IP, but not by DNS, and my local network still works.

As a temporary solution I entered my router's address in the VPN DNS Servers.