Please remember that VL 6.0 RC3 is late stage development/beta code. Problems are still to be expected.
Copy the /etc/xorg.conf from your VL 5.9 installation into VL 6.0 RC3 and see if that works.
Also, this post is in the wrong place for the developers to see it. It belongs in the RC3 bug thread in the Distro Development section of the forum. Can you please post there?