The nice thing about these is that you can make them any size. It may be some 10 years ago, but F&C had a series of articles by Robert Ingham, one of which had what I might call a "mini-Moxon", specifically for cleaning out the bottom line of d/tails absolutely square with the face. If all you want to do is cut dovetails, up to say about a maximum of 1-1/2" jaw gap, these can be made up from scrap MDF or ply. I made a couple very quickly for different projects, both using steel thread from one or other of the usual sheds. I used wing nuts for one, very successfully, and the fine thread for a drawer side of say 12mm. really isn't a problem. Robert Ingham brings precision engineering to cabinet making, and I think it is always worth seeing the jigs he makes for his work, even if you are not into his designs.
One of the things he uses on his dovetails vises, is a slave back board, again MDF or ply, behind the workpiece, taller than the bottom line of the d/tail. Slipping this behind the workpiece, after you have completed all the saw cuts, allows you to chisel to the base line, without risk of breakout.
Before you make one of these, I think it is worth giving some thought to the size of the jobs you are likely to want to do. If you are only likely to use it for say drawer sides, then a smaller size viz. Mr. Ingham, is worth considering, particularly if you have a small workshop. When not in use the "full" Moxon is somewhat cumbersome to store, and if you use steel bar, rather than the original wooden threads, can be a real pain.
I can't see a problem with a softer hardwood, because if the top of the front jaw wears, it can always be cleaned up square to the vertical jaw face, and reinforced with a Perspex or aluminium strip glued or screwed to the top face, to give a flat harder wearing surface to use as a register.