I tested in the current Lubuntu Vivid i386 daily (built from the newest desktop iso file)
Installed as you wished xubuntu-desktop and kubuntu-desktop
Then updated and dist-upgraded until apt-get was happy with everything up to date, and started testing:
ubuntu-software-center is simply software-center, and works already in Lubuntu (no reboot, no logout). I installed htop.
gksudo software-center
gksudo lubuntu-software-center also works in Lubuntu (no reboot, no logout). I installed xournal.
So it works now. I don't know if xubuntu-desktop or kubuntu-desktop or the final update and dist-upgrade made it work. But not from the menu: 'You have not the privileges to install ... PolicyKit ...'
Reboot
Lubuntu-software-center works from the Xubuntu menu (it asks for elevated permissions ...). I installed arandr.
Lubuntu still cannot install with Lubuntu-software-center from its menu after reboot.
Lubuntu-software-center works from the Kubuntu menu (it asks for elevated permissions ...). I installed smplayer.
I tested in the current Lubuntu Vivid i386 daily (built from the newest desktop iso file)
Installed as you wished xubuntu-desktop and kubuntu-desktop
Then updated and dist-upgraded until apt-get was happy with everything up to date, and started testing:
ubuntu- software- center is simply software-center, and works already in Lubuntu (no reboot, no logout). I installed htop.
gksudo software-center
gksudo lubuntu- software- center also works in Lubuntu (no reboot, no logout). I installed xournal.
So it works now. I don't know if xubuntu-desktop or kubuntu-desktop or the final update and dist-upgrade made it work. But not from the menu: 'You have not the privileges to install ... PolicyKit ...'
Reboot
Lubuntu- software- center works from the Xubuntu menu (it asks for elevated permissions ...). I installed arandr.
Lubuntu still cannot install with Lubuntu- software- center from its menu after reboot.
Lubuntu- software- center works from the Kubuntu menu (it asks for elevated permissions ...). I installed smplayer.