What to consider, Draw Calls or Vram Usage?

Hi we are working on a i phone game. But we have some perfomance issue in game. While playing game , it is breaking with memmory warning. Now we are optimizing our game. But we are facing a problem about the Draw Calls and Vram Usage. Which one affects most on performance??.. which is important to reduce??. waiting for your valuable answers.. Tanks in advance.