I've mounted my Emmert twice.
The "L" shaped plate does come off the assembly, and I recommend that additional step, to make sure you've got a proper fit before wrestling with the complete vise.
I use a pair of big Bessey clamps and a short stool in front of the bench to support the vise on the long screw to put mine on.
With the collar and mounting bracket as a template, you can set
the clearances pretty easily. I would NOT use a router on the narrow front of your bench - it's too easy to get beyond the depth you want
and have an uneven mounting surface.
For that, a handsaw and chisel should be enough.
Use the handsaw to score a series of kerfs to almost the thickness
of the front flange, and use a sharp chisel to get down to depth.
Remember that the vise face nearest the bench will be proud of your bench be an inch or so, unless you cut a deep recess for the PM vise.
That "inside jaw" need not be flush to the front of the bench, to have a perfectly useful application of the PM vise - few boatright shops or patternmaker's benches in foundries did.
https://www.youtube.com/watch?v=RGbmki3gs70