Nice bench design.
I like the ratchet mechanism on your leg vise. I've built a few of those, been using one I made out of aluminum for the last year.
I have the lever mechanism on the same side as you, and I would HIGHLY recommend you change it to the other side. You constantly have to move a step or two when you want to use the vise, and, while this doesn't sound like a lot, it is very inconvenient and drives me nuts.
Regarding the tenon/shoulder design, I wouldn't worry about it personally. That joint is very strong, easily tightened up if it loosens a bit, and it has held up very well on several large trestle tables and other furniture I've built. If you NEED to add a shoulder to the top I have done that once or twice by modifying the the wedge and design a little and having the wedge driven into a hidden pocket on the stretcher.