My rover is setup with a 24v/12v split system. The batteries run in parrallel for normal running/charging but kick into series for starting (accessories remain 12v during starting).
I'd love to be able to give you the name and model of the switching block which accomplishes this, but the stick on label has long ago faded and fallen off.

As for vacuum pumps/alternators, I forked over the equivalent of $US500 about a year ago for a 70 amp alternator with a vacuum pump on the back. Given the choice again I'd be mounting an electric vacuum pump right beside the brake booster and running a normal alternator which is much much cheaper.
Chopping a 24v alternator down to 12v leaves you with the same amps but only half the power. My vehicle used to have one and on full beam with the stereo going it couldn't keep up.