Update:

Via Aumha's Vista Support Forum:

Are you sure nothing happens?

PC World, Lincoln Spector wrote: Vista treats defragging as a hidden background task--no progress bar, no pretty boxes changing colors, no sign whatsoever that anything is happening. When you launch Defrag under normal conditions (by right-clicking your hard drive's icon in Computer, selecting Properties, and clicking Tools, Defragment Now), you get a dialog box for controlling when defrags happen, but that's about it.

Do the same thing in Safe Mode, and defragging quietly starts.

Here's another Vista Defrag problem: Entering defrag c: at a command prompt brings up an error message. In Vista you can launch a command-line defrag only from an administrator command prompt. To get one, select Start, All Programs, Accessories. Right-click the Command Prompt icon and select Run as administrator. You'll know you're on the right track when Vista makes you pass through a User Account Control dialog box (if you haven't disabled this security feature, that is).