On Mon, 11 Feb 2013 00:53:48 -0700
Found the answer: apparently updating Vuze this time also required
resetting file associations in the program. For anyone else coming
across this, this is what worked:
In Vuze go to Tools > Options > Interface
Click on the 'reset' button next to 'Reset explorer file
associations...'
Next time opening a magnet link, there'll be a box asking if you want
to associate that magnet file with Vuze. Check so to not see the box
again and then click on Proceed. Next time it will open Vuze straight to
the magnet box.