1. Log into XFCE session 2. Plug VGA cable in 3. Run settings for "Display" 4. Select second monitor 5. Set checkmark to enabled 6. Select first monitor, then second again (or the minotor spec option boxes are neither filled nor enabled, that's another bug) 7. Hit OK, no changes heppening. In contrast, lxde-base/lxrandr does not have any trouble with my setup. On a sidenote, I am mixing a 4:3 monitor with 16:10 wide screen notebook here.
You can move this to http://bugzilla.xfce.org/ as a) It's an upstream bug. b) I don't even have multiple screen setup to reproduce. Also missing emerge --info, preferably build.log (or emerge -pv output) for xfce4-settings, maybe xfconf and xfwm4 too to see USE flags used.
It seems to depend on the monitors involved - tested with another one - worked as expected. Closing.
