I recommend you contact Western Digital Support at:
WD - Service & Support Home by whatever means you prefer - phone is usually the best. Discuss the problem with them and see if they can assist you. You did not provide enough information about the make and model of the drive (there's a make name and the model is a MUCH longer code) for me to assist or research any further. If you do, I'll see what I can find, if anything. There are many possible causes - it is not compatible with Vista, it needs updated drivers or software, it is defective, - and it's also possible there's a hardware or software problem with the computer (but we don't have enough information to tell).
Do you have another computer (using Vista) you can test it on to see if it works? If not, it's probably the drive. If so, it's probably something about the computer. If you do this test and find it is the computer, post back.
I hope this helps.
Good luck!