set (real)VNC resolution on headless Pi 3B+

[Twist]

I'm unable to get realVNC to serve to a remote display in the resolution as set in /boot/config.txt
The 'usual' solution (modify /boot/config.txt) doesn't work...

Setup: 
RPi3B+ with freshly installed OpenPlotter v1.0.0


I've enabled VNC and SSH access. I've also enabled the OpenGL v4-kms-v3d driver.

With a physical HDMI screen attached all works fine. When I use a vncviewer, I get the exact same desktop. As expected.
When I disconnect the HDMI screen and boot the RPi Headless, vncserver always gives me 1152x864. I want to change this to something else.


I've set the usual modifications of the /boot/confg.txt file. most notably:

Code:

#hdmi_safe=1
hdmi_force_hotplug=1
hdmi_force_mode=1
hdmi_ignore_edid=0xa5000080
hdmi_group=2
hdmi_mode=9 # as well as several other modes, also from _group=1

Booting headlessly works fine, and when I log in via SSH and run tvservice -s to confirm the hdmi mode

Code:

pi@fluxpi:~ $ tvservice -s
state 0x120009 [HDMI DMT (9) RGB full 4:3], 800x600 @ 60.00Hz, progressive

vncserver and X are running:

Code:

pi@fluxpi:~ $ ps aux | grep X
root       539  2.0  5.1 139056 48692 tty7     Ssl+ 13:26   0:10 /usr/lib/xorg/Xorg :0 -seat seat0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
pi         856  0.0  0.0   4372   544 pts/0    S+   13:35   0:00 grep --color=auto X
pi@fluxpi:~ $ ps aux | grep vnc
root       460  0.0  0.2   5796  2716 ?        Ss   13:26   0:00 /usr/bin/vncserver-x11-serviced -fg
root       486  0.3  1.9  28824 18036 ?        S    13:26   0:01 /usr/bin/vncserver-x11-core -service
root       577  0.2  0.8  12496  8368 ?        S    13:26   0:01 /usr/bin/vncagent service 14
pi         858  0.0  0.0   4372   560 pts/0    S+   13:35   0:00 grep --color=auto vnc

But when I connect a vncclient it gets served a 1152x864 desktop  
 driving me mad. All the things I find online indicate it should work by modifying the config,txt