Sometimes a double Screw is fixed to the side of the Bench, as at g; or sometimes its farther cheek is laid an edge upon the flat of the Bench, and fastened with an Hold-fast, or, sometimes, two on the Bench.
Christopher Schwarz reintroduced us to the latter alternative, the Moxon twin screw vise as it is now known. Here is a link to a post on his blog at Popular Woodworking and here is one to the Sketchup model. This vise has become very popular and appears to be on its way to widespread adoption. I made a crude prototype and quickly decided I would never be without one. It is because I like this vise so much that I am not putting a vise on my bench.
Making your twin screw vise portable has numerous advantages. The experts want you to make your bench low, primarily for planing. Like many, I resisted this advice and finally settled on 34," too high by their standards and too low by mine. This really came home to me when I saw a picture of Christopher Schwarz, a tall fellow, dovetailing at his bench. He looked like a giraffe talking to a child in a cartoon. He was bent way forward at the waist and way back at the neck. It is no wonder that he embraced the Moxon vise. Having the vise sit on top of the bench means the work is raised up considerably to a much more comfortable position, yet still held securely.
Another advantage is its portability. I needed to do something for a friend recently at his house. I took my prototype Moxon vise and my sawbench with me and used the vise on top of the sawbench. It worked great. I have seen pictures of it being used on a picnic table.
The rub is figuring out how to make or buy the screws. There is a manual threader available but it is generally known to be of highly uneven quality. Beall makes a threader kit that is somewhat more expensive and uses a router. Both of these options require high quality dowels, which are hard to come by in large sizes (1 1/4 or 1 1/2"), though Beall does sell dowels on its website, or you can make your own. An interesting alternative recently came to my attention. The December 2011 edition of ShopNotes has a project for a different kind of benchtop vise. They do a nice job of making a screw from acme threaded rod and a shopmade wooden hub, which doesn't require a lathe for construction and is very rugged. You could make two of these for a twin screw vise.
Finally, a variant of the twin screw vise is to make it as part of a small "bench on bench" for joinery, an idea that is also gaining popularity. I made a crude prototype of this too and really like it.
The Moxon vise lets me lower my bench height and feel comfortable not having a vise on my bench. I am reassured by Bob Rozaieski comments below that my direction is viable.
Update: See the comments for several approaches using steel screws. McMaster-Carr is a good source for acme threaded rod. For example, 1"-5 tpi rod 12" long can be had for as little as $11.50 apiece. See this page. If you prefer wooden threads, many major suppliers carry the manual threader kit. This is the link for the router-based threading kit, which is also carried by Lee Valley.
Here is a link to Derek Cohen's version, with an I beam for dovetailing that he devised.