I have seen many examples of different kinds of workbenches and instructions for building them online, and when they feature a face vise it is nearly always positioned at the left hand of the bench. Usually, no reason for this is given, but I have seen reasoning that the vise should be on the right for two reasons (sorry, read this a long time ago and I don't have a link): 1. when cross-cutting wood by hand, the board sticks out past the left hand side of the bench, you saw it with your right hand (if right handed) and then you can hold on to the sticking out part with your left hand. 2. You spend most of your time standing in front of the vise, and if right handed you can place your tools on the bench within easy reach.
My bench (which was left by previous owners of my house) is in the back left corner of my garage, so that the left hand side is against a wall and the right hand side is free. Due to the position of doors, this cannot easily be reversed. I hope one day to build a bench which can be raised up on casters, but for now it is what it is. I have been happily using it with a small vise attached towards the right hand side, as the left is blocked by a wall. Both the points above are slight annoyances, but no more than that.
I have now acquired a much bigger vise (old Record 52 1/2). To fit this will require some modification to the bench structure. Before I do that, I want to check - what is the best position to put a vise if I can't put it on the left hand edge?
It's probably an obvious question - if I can't put it on the left, I have to put it on the right. But, I want to check in case there is some advantage to putting it in the middle, for example, or something else I haven't thought of.