It may be that xorg is not autodetecting the correct resolution of your 2nd display. You can use xrandr to duplicate the image or set a specific resolution on the vga port.
Common display names are:
LVDS - Laptop Screen
HDMI - High-Definition Multimedia Interface
DVI - Digital Visual Interface
DP - DisplayPort
VGA - Video Graphics Array
TV - Composite Video
S-video - S-Video
Get root access: https://github.com/recalbox/recalbox-os/wiki/Root-access-on-terminal-(EN)
run xrandr from terminal to see what your displays are: (my example laptop)
~ $ xrandr
VGA-0 connected 1280x1024+0+0 (normal left inverted right x axis y axis) 340mm x 270mm
1280x1024 75.02+ 60.02
1024x768 75.03 70.07 60.00
800x600 75.00 72.19 60.32 56.25
640x480 75.00 72.81 59.94
LVDS-0 connected primary 1440x900+0+0 (normal left inverted right x axis y axis) 367mm x 230mm
1440x900 59.94+
HDMI-0 disconnected (normal left inverted right x axis y axis)
Once you know what your display names are you can use xrandr to set your display.
See the example displays I used with my laptop with a monitor connected via VGA, your display name may be different.
xrandr --output LVDS-0 --auto --output VGA-0 --auto --same-as LVDS-0
(this will duplicate the same resolution on the VGA port)
To set a specific resolution on the vga port (eg: 720 resolution)
xrandr --output VGA-0 --mode 1280x720
A more complete command that will duplicate the display and set set VGA to 720:
xrandr --output VGA-0 --mode 1280x720 --same-as LVDS-0
You could also switch off the laptops display if you want:
xrandr --output LVDS-0 --off
Or have the laptop screen off and VGA screen on:
xrandr --output LVDS-0 --off --output VGA-0 --mode 1280x720
Once you happy with it you can make it permanent by adding it ~/.xinitrc
https://github.com/recalbox/recalbox-os/wiki/Change-display-resolution-on-x86---x86_64)
My apologies for waffling on, hope it may help
Sources:
https://wiki.archlinux.org/index.php/Xrandr
https://wiki.archlinux.org/index.php/multihead