How to switch from Xvnc to Xvfb-run?

Question

linux

The program is launched by communicating with the X server. An example startup script using Xvnc:
Xvnc -ac :100&
export DISPLAY=":100"
sleep 1
./programm
killall -9

Xvnc run) as a replacement for Xvnc.

However, it was not possible to deal with it. I ask for advice, what is wrong in my new script, which just hangs at the programm runtime:

#Xvfb :100 -screen 0 1024x768x24 -fbdir /home/user&
sleep 2
export DISPLAY=localhost:100.0
xvfb-run -n 100 -l - a -e /home/user/err.txt ./programm
killall -9 xvfb
killall -9 xvfb-run

Do I need to start Xvfb before running xvfb-run (the line is now commented out)?