They're called "T-slot nuts". Most machine shop supply places sell them. They can be used either with studs, nuts and washers, or bolts and washers from the top side. They're also used on milling machines, lathe face plates, and other workpiece mounting applications. Try checking here:

www.victornet.com

http://www.victornet.com/alphabetic/T-Slot-Nuts/791.html

These kits come with nuts, studs, and clamp blocks- - -easier and more versatile than using the slots in the vise base.

http://www.victornet.com/alphabetic/Clamping-Kits/46.html

I buy most of my machine shop supplies there.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!