Gary,
I use my tail vise a lot more than my shoulder vise. The vise faces I most often use are those on the tail vise.
Like David, I did have trouble with the heavily cantilevered tail vise sagging on the first bench I built. Since then I built four more benches and modified the design to eliminate the sag. I moved the legs at the tail back under the vise, it's easy to do. Two of the benches I built are portable and have a tail vise on each side so two people can work on one bench.
I do find the Frid/Klausz shoulder vise gets in the way at times because it sticks out so far. If I was building one again, I just use something like the old Record quick release vise. The shoulder vise I have is great for supporting a variety of fixtures though.
Yes, you could move the legs to the edge but I'm not sure of why this is needed. If you do, it's important to remember to keep the stretcher back away from the tail vise so that things clamped in the jaws can go all the way to the floor. If you're looking to install a sliding deadman it wouldn't be a difficult modification. I find I don't need that. For supporting longer stock in the shoulder vise, I just clamp a 2 X 4 vertically in the tail vise. This 2 X 4 is the same length of the height of the bench and has holes drilled a couple inches apart in its edge. I insert a short dowel in the appropriate hole to support the end of longer stock.
I might use Klausz's plans as a guideline but there are a number of changes I'd recommend. One other important one is the shape of the small land inside the bench dog holes. If you leave that parallel to the bench top as both Klausz and Frid show debris collects there and makes it impossible to set dogs level to the top. just angling that land at about 45º lets you keep that land clean by just removing and reinserting the dog.
I'm attaching photos of one of our portable benches with 2 tail vises and my bigger bench that I've been working on for around 15 years.