Coming in late here, but want to make a couple of comments.

I'll announce up front that I'm not a fan of leg vises, myself. I struggled with one as a kid, so that has a lot to do with my attitude. Granted, that leg vise was pretty crude, as the old bloke was always flat out & often cobbled up something in 5 minutes to get the job done, then never had the time or inclination to go back & 'do it properly' (hmmm, thinking about it, I think it might be genetic, not just a time thing ). The leg vise worked, but at some point an old Dawn appeared on the scene, and was duly installed. The Dawn was a far better gadget to use, until the QR nut stripped.

So, I'm prejudiced, but I still can't see that a leg vise has any advantage whatever, other than simple construction (until you go & complicate it with all sorts of gadgetry ), & that was their attraction for our forefathers. When making the bench I have now, my preference would have been for a shoulder vise, because of their slightly greater versatility, but they do stick out a looong way, & lack of space & a protective attitude to certain parts of my anatomy induced me to go with a more 'ordinary' arrangement. I came up with my own vise, based on a metal tail-vise screw. This vise holds wide boards for edge-work quite adequately, paired with a sliding support: Support1.jpg

The board is held outboard of the left guide bar, and granted, this applies a bit of wracking force, but it doesn't take much pressure to hold even a heavy board (I've had bench-tops in there), and the wracking is countered by having wide-spaced guides, and a second pair of stationary guides under the bench (all the guides are 3/4" water pipe): Front vise red.jpg
The bench & vise have had 30 years or more use, now, and both are travelling well.This vise was a bit more complicated to make than a basic leg vise, but easier to use. You have to go up-market & buy an expensive system for controlling the adjustment of a leg vise, to get the same simplicity of operation.

Twin-screw vises are an interesting concept, but seem to me to be more bother than they are worth to have as your 'regular' vise. For the few times you need that sort of clamping, an auxiliary vise like this d_tail vise.jpg (which I made after I saw Derek's post on his "Moxon vise" ), is the bees' knees. Not only does it get the work up higher where I have more chance of seeing layout lines, & it's more comfortable to saw dovetails, etc., it can be put out of the way when not in use.

I could live with a (decent) leg-vise, or just about any form of front vise, if I had to, because I don't use them all that much. What I would never be without is a tail vise! A travelling dog system is a good start, and easier to install, but you don't get the full bottle with a travelling dog. Every good bench deserves a tail vise.....

Cheers,