There are a couple of things here, Ken. When the bench gets narrower, and the vise is out, and if it’s a large vise, the bench will hold fine but if you are on pull strokes, the narrower bench will tend to be levered toward you. It’s not a problem if you know this but can be if you are caught by surprise. I usually have a shelf across the rails and have the shelf loaded and this counters it anyway. Going three inches narrower will work fine but be conscious if this detail. If you are against a wall there is generally no issue.