word of advise. often times the bands' merch doesnt arrive until the band does, so your best bet is heading over there a tiny bit before their set if you dont see their stuff there earlier. and often times it sells out. just sayin
God, I love the vendor tents at Coachella. I leave without about 4 or 5 shirts each year.
Wait until the very end of Sunday night and most vendors will drop prices like crazy. In 04 I got a $40 shirt for $5! Band merch on the other hand goes really really fast. You better grap that shit up fast unless you're a XXL.