I recommend my own posts in this thread: http://www.veganfitness.net/forum/viewt ... hlight=b12
It's a very long thread, but if you just read my messages (not trying to discourage you of reading it all), you'll get a pretty good overall picture of B12 with lots of valuable information.
Short answer to your question however would be: drop the multivitamin if you eat a halfway proper vegan diet and take B12 instead, 2000µg once
a week is the simplest method. More in that thread.