It looks like the one that goes to the leak detection pump in the rear fender. There should be a hose that connects it to another hard line at the edge of the engine compartment. There is a little panel that has the 2 fuel lines and 2 evap lines on it. See this DIY I put together on the EVAP system:
http://newbeetle.org/forums/2-0-liter-gas/59573-diy-evap-system-leak-checks.html
http://newbeetle.org/forums/2-0-liter-gas/59573-diy-evap-system-leak-checks.html