It would appear that Tasker and a plugin can be used to do this, humn, only slight worry there would be whether it's possible to get data to leak while the VPN is not up...and whether or not it leaks if the VPN goes down and it has to bring it back up again.

Not even sure how you'd go about testing that!

Adrian

Edit:

Humn....maybe not, seems that the VPN plugin is not compatible with Android 4.x....