I am building a bench, starting with two vices, but I have been thinking of adding another vise. I have an end vise and shoulder vise - hardware has been purchased, but not installed because I was interupted by a paying job for a friend. Once I have some experience with these two vises, there may be a need for a third.
I agree with the others that height is important. My bench is 36" high for making cabinets (I am 5'6" tall or short, depending on your point of view). It really works well. The height was determined from butchering venison - at the kitchen table, it kills my lower back because of the slight bend. At the kitchen cabinets, I can do this work all day and not be bothered.
I will probably build a smaller bench if I need a bench shorter. If my existing bench needs to be higher, I can raise the top with spacer blocks. I'll have to make the spacer blocks, but i will cross that bridge when I come to it.
Rich (the Yooper)
"To the world, you may be one person, but to one person, you may be the world."
"Common sense is not so common."