I would first make sure that the BIOS has all of the CPUs enabled. ( sounds silly but baby steps )
did your motherboard come with any type of CDROM?

I have a feeling that Vista has a HAL (hardware abstraction layer) installed that is dedicated to dual core so it can't see more cores.
In a perfect world the plug & play should have detected the newer CPU/motherboard and tried to install a new HAL.
You should be able to upgrade the HAL although I'm not 100% sure how it's done.
You can try to use the device or hardware manager to upgrade your CPU driver or motherboard ( and thus get a newer HAL installed ).

and as someone else said, you can always start from scratch and re-install vista.
good luck