I;ve always had the vise mtd on the left end of the bench w/ a little room to spare so it's not at the extreme end, I'm right handed.
I can't seem to find it comfortable to have the work hanging out of the vise pointing to the left and try to do too much on it.
I just built another bench in my upstairs workshop. A smaller bench, less room around it and I placed the vise a little more towards the center,,but not much. That was because of the lack of room to the left of the vise,,I wanted to allow for some room to slide the longer stocks and bbls through.
Putting it in the middle just throws me all off!,,I need that room to work and make a big hill of tools;
The 2 gun factorys and several gun shops I've worked in doing repair all had the vise mounted on the left end of the benches. Probably where I got the reinforcement for it from. But one guy in the last shop I worked insisted on having his moved to the right end of his bench.
No real correct answer,,just whatever is comfortable for you.