Stick's advice is good. I would definitely add a 1/4 inch Masonite on the top, screwed in place, not glued, so you can replace it easily. If you are going to use that lip for clamping (as I would and do), then I would seriously consider gluing a second edge of good ply or even better, a good flat 3/4 to 1 inch pine under the top. The extra thickness will support the vise very well and keep the edge flat. I would actually glue this on and add a few screws to lock it in place while the glue dries. The edge of my bench is almost 4 inches thick, using the method I described, and it's both flat and strong.
My table is from Harbor Freight, so the top is actually 3/4 thick with about a 3 inch Maple lip around the top. I used 1 inch pine to build up the edge, so the pine I placed under that lip is about 2 1/4 inches wide. Because the lip is really thick (three layers), I was able to put a fold down section behind it. Because the table has drawers directly under the top, I didn't add a front vise. But today I'd sacrifice one drawer to put a vise there. A front vise is wonderful for working with a hand plane. So your thickening the edge can be selectively extended under the top to allow you to add a front vise in the future. Adding a few strips of hardwood as trusses will give you years of extra service.
The more I do, the less I accomplish.