Time to repurpose the x64 dev server as a development workstation. I expected to have trouble with the RAID drivers; I've always had trouble with VIA's RAID drivers for the VT8237... but I was happy to find the VT8237 drivers for Vista x64. And they worked great!
But Vista wouldn't install anyway... *pout* Because "Windows is unable to find a system volume that meets its criteria for installation." For the love of all that's Assembly language, why?!
Buried in this Vista RC1: Unable to find a system volume (TechNet Forums) post was the clue. Vista doesn't like having anything else between it and the boot partition. Making the RAID1 array (the onchip controller) the first bootable device instead of the floppy or the DVD drive was the trick!
"Success is the ability to move from one failure to the next with enthusiasm." - Winston Churchill
On toward the next...