One thing Vidic has over Stam is goalscoring.
Jaap scored just the 1 goal for us while he was here. Considering his size and the quality of crosses coming in, you would have expected a couple per season.
Vidic must have scored about 5 goals already by now, and often important ones (I can remember an equaliser at Wigan, equaliser vs Benfica, winner at Everton...) and that only adds to his charm.
While both players were aggressive on the pitch, it came across in different ways. Vidic flies into tackles and leaves his opponents in a heap, whereas Stam usually just out-muscled them - I remember him brushing Shearer off the ball with arrogant ease... Old Trafford loved it! Also the famous incident with Zamorano when he wouldn't let the Chilean barge into him, standing his ground with hilarious consequences.