Had the problem, that Fallout1 from Steam always renders on my right monitor (the primary one). Problem with this is, that I wanted to play on the left monitor, but no common key shortcuts worked to move it onto the other screen. I solved this problem by editing following lines in f1_res.ini within the steam common folder:
WINDOWED=1
WINDOWED_FULLSCREEN=1
these options might be available within the options menu. But i was not able to find them.