I never understood why they shot Vito because of what Sonny said during the Vito-Sollozzo negotiations. I tought sonny said, angrily, "then you go tell the Tattaglia's that their guarantees...." I couldn't hear it was actually a question, out of interest. And thus, I didn't understand why everyone was so mad at Sonny.
Kinda stupid of mine, because this is a major plotline.