I had some trouble to setup my old AOC 912Vwa VGA monitor on My Raspberry Pi 3.
So I decided to write it down here, and maybe help some folks ;)

I'm using RECALBOX.

First you'll need an adapter - VGA to HDMI. (Sorry, there's no magic for the hardware part)

Then find out the config.txt file, on BOOT partition.
Backup its content.

Then set those configurations and reboot your Rasp.

hdmi_safe=0
hdmi_mode=16
hdmi_group=2
disable_overscan=0
hdmi_force_hotplug=1
disable_overscan=1
overscan_left=0
overscan_right=0
overscan_top=0
overscan_bottom=0
program_serial_random=1
config_hdmi_boost=4
hdmi_ignore_edid=0xa5000080
temp_limit=85
force_pwm_open=1
hdmi_drive=2
dtparam=audio=on
boot_delay=3
disable_splash=1
cec_osd_name=recalbox
gpu_mem_256=128
gpu_mem_512=256
gpu_mem_1024=512
kernel=zImage

Hope it woks for you too!