The default config file is '/etc/X11/xorg.conf'. The '/etc/X11/xorg.conf.vesa' file is just a sample, which would have to be copied first to '/etc/X11/xorg.conf' before any changes are made on that copy.
Or was that what you did and only quoted the wrong file name in the OP?
EDIT 1: I believe that the entries of the config file are being quoted in '/var/log/Xorg.0.log', so have a look there for (a) what config file is being picked up and (b) whether the intended option appears or not.
EDIT 2: Following my own advise I can now confirm that having the following few lines
Section "ServerFlags"
Option "DontZap" "true"
EndSection
in '/etc/X11/xorg.conf' disables the default behaviour of 'Xorg-7.5' to allow 'Ctrl-Alt-Backspace'. As this is IIRC a reversal of the default from 'Xorg-7.4' maybe the .info file should point that out.