EDIT: I'm sorry if this is a critical detail but i totally forgot to say that i have added nomodeset to the kernel options in /etc/default/grub.
Then you cannot power off the screen, because with nomodeset you have disabled most of your GPU driver, together with all of the output controls that it normally provides.
setterm should work; however, the way it works is by performing special operations on its input terminal, so if you need to use it from SSH or from a service, you must specifically redirect its input and output to one of the "console" terminals. In addition to that, sometimes it uses Linux-specific ioctl calls but sometimes it uses terminal-level "ANSI codes", so you also need to override TERM with one that would be appropriate for the Linux console (instead of whatever you get via SSH).
# TERM=linux setterm --blank force </dev/tty1 >/dev/tty1
ExecStart=setterm --blank force
# (StandardInput=file:/dev/tty1 may actually work better.)
If this doesn't turn off the screen, try adding
You can also completely disable the video output at kernel level; boot with the kernel option
video=LVDS-1:D to disable the output named 'LVDS-1'. (Check /sys/class/drm to find the output names.)
vbetool is not likely to work anymore, as it functioned by directly calling into the "Video BIOS" and that's not how modern systems do things. (Even then it kind of bypassed the OS and sometimes would end up confusing it.)