Bitter end on the "bottom", not attached to the boom, definitely. While it may be easier to grab the bitter end when it's hanging down, there is no question that you have much more leverage and can use your legs when pulling up on the line than pulling down. This does require you to go on deck to adjust the vang, but I think that is what is necessary to make the vang effective. I've tried it both ways, and that's what I've found on my Oday 23. YMMV.
BTW, I agree with Dave on the twist shackles; I bought a couple of second hand snap shackles for either end of my vang, and it makes it much easier to stand off to the side while hauling on the line.