It's important to note that Vito assumed Tattaglia killed Sonny because tessio's crew had killed Phillip Tattaglia's son, Bruno.

Bruno Tattaglia kills Luca Bratza and then disappears from the film. He is killed off-screen soon thereafter as Tessio notes "We hit Bruno Tattaglia at 4." In the book Bruno Tattaglia is killed by being thrown into an oven.

Vito naturally assumed that Phillip Tattaglia killed Sonny to avenge Bruno's death- but as the meeting plays out he of course comes to realize it was Barazini.