NOTE: this is some old code that I haven't touched in a couple years, and may have not worked completely when I originally wrote it. That being said, there is no API for managing VPN connection in OS X. This is a good starting point for anyone who needs to manage VPN connections programmatically.
ObjC Library for Creating/Editing/Deleting VPN Connections. Simple objc API that interfaces with some GUI scripting.
GUI scripting has become harder with Mavericks. Check out this script for an easy way to enable GUI scripting.
This is great for automatically updating a VPN connection information on a bunch of machines that you manage.