/usr/share/X11/vtwm/system.vtwmrc

copy this to your home ~/.vtwmrc

Use the vtwm manual 'man vtwm'

Either copy it to you home directory as vtwm will read it as default
or use the -f option to read a different one ex: 'vtwm -f ~/.vtwmrc.dark'
vtwm by default will read ~/.vtwnrc unless the -f option is used.

More ideas on vtwm:
https://cs.earlham.edu/~jrogers/classes/cs310.06f/vtwm_tutorial/
http://www.vtwm.org/vtwm-manpage.html
https://www.freebsd.org/cgi/man.cgi?query=vtwm
http://dotshare.it/dots/8469/

Best of all sample .vtwmrc files and screenshots
http://www.vtwm.org/vtwm-and.html