Use Disk Utility => Partitions tab (you need to click on the raw hard drive name) and change the number of partitions, click "Optons" at the bottom and change it to GUID (GPT) and click Apply.
Then Apply the partition number, name, size again on the screen you were on.
Rather simply when you see a screen shot from Partition in DU.
Nothing else to do though.
Vista came out almost 7 yrs ago though. So not sure how reliable or good the drive is. I would while in Windows run WD Llifeguard and zero out the drive which will remove all the partitions and partition table, do a full format to insure no bad blocks. But compared to newer drives in terms of capacity and performance... unless you have been updating drives already, I'd use it for a spare backup.