Probably I don’t understood you question correctly but VNC has nothing to do with a connect monitor via HDMI or other resolution settings. For VNC you don’t need to install any desktop like Mate. It’s just needed to have a VNC server running and the appropriate VNC client software on your desktop computer.
How it works is described in this post
https://dietpi.com/forum/t/resolved-removing-diversion-of-boot-overlays-readme-err/74/1
Desktop resolution settings for VNC can be found in DietPi config file “/DietPi/dietpi.txt”
Check for section
#------------------------------------------------------------------------------------------------------
# D I E T - P I
# DietPi-Software settings
#------------------------------------------------------------------------------------------------------
there you will have
#VNC Server Options
SOFTWARE_VNCSERVER_WIDTH=1280
SOFTWARE_VNCSERVER_HEIGHT=720
SOFTWARE_VNCSERVER_DEPTH=16