After some research I actually managed to get my Dac-2 to work under Vista x64. I had to force the installation of generic drivers from the communication chip manufacurer FTDI (
http://www.ftdichip.com). New drivers (version 2.04.06) for Vista x86 and Vista x64 were released on 20th March 2008.
I guess the only work needed to get the drivers official is for the developers at Digital 1 to alter the PID of the INF file and supply a digital signature for the driver package. But if the DAC-2 is not supported anymore, then I guess we need to do a messy installation.
By the way, my DAC-2 now works flawlessly with PCDJ Reflex on Vista x64.