I'm good friends with the person that designed the Vac Rac line. The limiters are one of my favorites, but the preamps kick some serious a** as well. They at one point had a 10 channel box of preamps in the line that are very cool to bring out on gigs-- 4 spaces for the pres and 3 for the power supply.
To answer your questions, though: The Vac Rac line is no longer being manufactured. However, if you contact Steve Firlotte, who made them, he does occasionally do short runs when enough order pile up. The units he makes now are not the modular system, but rather stand alone 2 channel boxes with the power supply built in. It is unfortunate that they never did particularly well commercially because they do sound so good.
I don't know if they still have it, but Coast Recording supply in Hollywood had a single 4-module rack left that would make a hell of a vocal channel (or anything else...). It had a preamp, limiter, EQ, and ?? (I can't remember what the 4th module was, but perhaps a DI...). I think they wanted about $4500 for it.
--Ben