I can remember a tutorial by TripleX, though I may be wrong.
Anyway, the best way to learn those commands is by making a small test level (big hollow cube, default texture) and mess around with 2 entities(like warlock.mdl) using those vec_*_* commands. You can then actually see what happens and you'll understand it better.
Also it can be helpful making a panel that displays the position(x,y,z) and the angles(pan,tilt,roll) of both entities aswell as of the TEMP vector. you can then also see how some variables change(as you do not always get a visible result)