We are talking about a milling table or a table on another machine tool, aren't we?
First some milling vises are intended for mounting the long way. This is usually a two piece style vise which allows very long pieces to be mounted between the jaws. But "very long" is still a significant amount shorter than the table's length.
The biggest thing I can see with a vise mounted the long way is the screw handle will hit the table: you can't do a rapid movement by spinning it with your finger. That wastes time and wasted time can be money in a commercial shop. You also can not put one of those three armed speed handles on them. Things work a lot faster with the handle out in open space.
I went through a process to determine the best size vise for my mill. Many advised me to get a 6". I suspect that is more or less the default size that most machinists think about. And there are a lot of used 6" ones out there.
But, in addition to asking for opinions, I also made some cardboard cutouts that showed the outlines, the location of the fixed jaws, and the mounting holes/slots as well as I could manage from the catalog specs and photos. For my mill it came down to either a 5" or a 6" size. When I placed the two cutouts on my table, with the mounting slots over one of the table's Tee slots I quickly saw that the 6" looked oversized. I tried some table movements and found that there was no way that I could reach all of the jaw's gripping area with a tool in the quill. So much of the extra capacity of the 6" was just wasted. But the 5" allow full access to it's entire area between the jaws and even some space behind the fixed jaw for things like edge finders to work in. The 5" was perfect for my mill and I saved a significant amount on the price. In fact I would up buying two 5" vises so I could mount longer work pieces using both of them. This came to more than a 6", but the work envelope was a lot larger then the 6" and I could reach all of it with a tool. Win, win, WIN.
And with one or two 5" vises on the table, with their noses hanging over the forward table edge, I can hold work pieces that are only limited by the width of my shop, not the length of the table. I can only work on a foot or so of them at one time, but items many feet long are a definite possibility. That could not happen with a vise mounted the long way.
You can see that discussion here if you wish:
Milling Vise Size? -
The Home Shop Machinist & Machinist's Workshop Magazine's BBS
