No one every had regrets about making a bench TOO HEAVY. Don't skimp if you plan on doing heavy work on it at any time.
On mounting the vise be sure it's located as directly over a good sturdy leg as you can instead of out on the top. And even if the other legs are only one piece of 2x4 make the leg under the vise out of at LEAST two. Three would be better. And even better would be three laminations of 2x6. If you can't fit nuts and washers on the lower side because of the leg being in the way than use lag screws. The key thing is that you want the vise well supported by the sturdy oversize leg so that any hammer work is connected directly through the leg to the floor. Mounting the vise out on the top away from the leg means you may as well mount it to a trampoline.
Nothing wrong with gluing and screwing together some pretty rough lumber to make up some mass, and then surfacing it with something nicer too.
Another suggestion, is to plan around some bracketry to anchor the bench in place if you are not making it into the 'immovable object'. How you go about it is going to depend a LOT on your situation.
I framed my benches up with 2x4s, used 4x4s for the legs, and 3/4 MDF for the tops and lower shelves. Plywood for some others. Lucked into some paper faced form plywood that worked very well.
I left enough room under mine to fit a pallet dolly, so I can move these latest ones. The last two sets of garage workbenches were requested by the purchasers of our houses, so they stayed behind and I built new, on pretty much the same plan, other than raising the lower deck so they could be moved easily if required.
If you want a really stiff and flat bench top, look at how doors are made. Putting a layer of ply down, ripping a 2x4 into strips to form a lattice or honeycomb spacer with, then adding a thinner ply layer to the underside, will make a far stiffer, but movable bench top, should you wish.
Again, going to depend on your capabilities, and your need.
Worth digging around places like the garagejournal site and other web locations, to see what has been done and how.
Putting the vise on a corner gives the ability to approach he work from two sides without having to adjust the vise or the work position in it.
If you dig around, there are ergonomics studies that tell you pretty much where the height should work out based on various landmarks of your own body. FWIW, I like having at least one vise up a fair bit higher, for when I am doing detail filing and such. An adjustable height stool is pretty nice too!
Cheers
Trev