A small script to quit Transmission as soon as the VPN connection is down.

StartTransmissionVpn.AppleScript

on idle
  tell application "System Events"
		tell current location of network preferences
			
			set vpnConnection to the service "VPN"
			
			-- Check if Transmission is running
			tell application "System Events"
				set transmissionIsRunning to (count of (every process whose name is "Transmission")) > 0
			end tell
			
			-- Check if VPN is down
			if current configuration of vpnConnection is not connected then
				if (transmissionIsRunning = true) then
					set app_name to "Transmission"
					do shell script "killall " & app_name
				end if
				connect vpnConnection
			-- Check if VPN is up
			else if current configuration of vpnConnection is connected then
				if (transmissionIsRunning = false) then
					tell application "Transmission" to activate
				end if
			end if
		end tell
		return 10
	end tell
end idle