Women.

They talked among themselves. Maybe one of Fanucci's girls or his wife saw the murder but knew better than to say anything. But girls being girls, they talk and then it spreads through the streets.

Or maybe not.

Maybe Vito talked with Maranzalla out of respect and received his blessing to continue working the district in exchange for ridding the neighborhood of a rat and maybe a tribute to The Black Hand. And things just fall into place after that.