The documentation for ZENworks 11sp2 and 11sp3 notes that remote management via VNC is different for machines running OS X 10.7 and later. Specifically, on these later machines VNC will start a new login rather than connecting to the user's currently logged in session. The ZEN docs suggest using Vine server as a substitute for Apple's built-in VNC server for OS X.
However, it turns out that you can easily restore the previous behavior by modifying an OS X preference. The command to issue to the Terminal is as follows (it should all be on one line):
This command can be deployed via the workstation Terminal or via ZENworks bundle script. Once it is issued, VNC connections to the target workstation will always connect to the currently logged-in user. This eliminates the need for Vine.