Depending on how much paint you need, I usually use a Testors Paint marker in red.
You can find them at any hobby shop or even WalMart. Depends on how fancy you want to get with your touchup but it's an enamel and is very convienient. My bike isn't winning any concours anytime soon so it works fine for me.
BTW, the VW color is Tornado Red. If you search dilligently enough, you can get the paint code or just order the appropriate touch-up in pen form, bottle or spray can from any dealer. I know it isn't exact either, but close enough.
-Randy