This should work for you.
Open up Firefox and its preferences window. Go to the "Applications" tab and you should see "magnet" on the list. Click on "Always ask" and if Transmission does not appear as an option then click on "Use other" and navigate to /usr/bin on your file system. Look for "transmission" or "transmission-gtk" and click that.
12.04 - How to make Firefox open magnet-links in Transmission - Ask Ubuntu
Just replace transmission with Vuze in your case.
Open up Firefox and its preferences window. Go to the "Applications" tab and you should see "magnet" on the list. Click on "Always ask" and if Transmission does not appear as an option then click on "Use other" and navigate to /usr/bin on your file system. Look for "transmission" or "transmission-gtk" and click that.
12.04 - How to make Firefox open magnet-links in Transmission - Ask Ubuntu
Just replace transmission with Vuze in your case.