Proper fixing hook in vice’s jaws, is what we begin with when we tie a new fly.
Wrongly fixed hook may damage the jaws (break off or deform their tip under very strong pressure). if you are using a Regal-type vice this is possible that the hook will pop up (I have experienced with that at the beginning of my journey with fly-tying, fortunately the hook hit me in the forehead, not in the eye).
Hook placement is very good example.