http://ubuntuforums.org/archive/index.php/t-1771058.html

I accidentally discovered a fix for this while trying to solve a different problem.

edit
~/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-keyboard-shortcuts.xml

find the line 

<property name="&lt;Super&gt;Tab" type="string" value="http://www.mamicode.com/switch_window_key"/>

and change it to 

<property name="&lt;Super&gt;Tab" type="empty"/>

reboot or whatever and then tab will work properly!

I have no idea why but when using vnc this file seems to override tab‘s normal behaviour and makes it into a switch window key.