Well, in fact half-wrong.
(1) any filename ending in .conf in /etc/X11/xorg.conf.d will be read by the X server.
(2) but in /etc/X11/ only xorg.conf will be read.
I should have RTFM as man xorg.conf says that under the DESCRIPTION title:
" Xorg uses a configuration file called xorg.conf and files ending in the suffix .conf from the directory xorg.conf.d for its initial setup."
So you could use either one of these commands:
cp /etc/X11/xorg.conf-vesa /etc/X11/xorg.conf
cp /etc/X11/xorg.conf-vesa /etc/X11/xorg.conf.d/xorg-vesa.conf