I’ve got a couple of USB<->Serial adaptors, neither of which work in Vista. On plugging them in, I went to the hardware manager, and looked at the properties of the unknown device (specifically the HardwareId) - this showed up as USB\VID_0607B&PID=2301.
A quick search on the internet shows Prolific as the vendor ID 0607B, and on visiting their website, found out that they do not support Vista, and you need to go to whoever you bought your adaptors from.
My adaptors are unbranded ones, so I simply searched for another vendor with an adaptor that is Vista compatible, and uses the Prolific chipset.
I downloaded and installed the Targus PA088E drivers for Vista, installed them, and then uninstalled (this left the driver sys files behind), and I now have two working USB<->Serial adaptors under Vista.