On Mac OS X <= 10.10, I could run the following command to open a VPN connection window:
function go-vpn { /usr/bin/env osascript <<-EOF tell applicatio
VPN="YOUR_VPN_NAME" IS_CONNECTED=$(test -z `scutil --nc status "$VPN" | grep Connected` && echo 0 || echo 1); if [ $IS_CONNECTED = 1 ]; then scutil --nc stop "$VPN" else scutil --nc start "$VPN" fi