Get out the CD that came with your mobo and run its Driver Install utility to find and install the RAID drivers for your mobo in vista. Make sure you choose the drivers for Vista, not for a different Windows.
This will allow Vista which boots from a third non-RAID drive to use the new RAID array for data storage etc. It will not allow you to boot from the RAID array. For that you would need to re-install with appropriate driver installation.