Yes. An attempt already had been made on Michael's life in Sicily. Vito was certain that, absent some conciliatory action on his part, Michael would be dead meat when he got to America. Barzini or one of the other dons easily could have arranged for the police to arrest him on suspicion of killing McCluskey and Sollozzo, and then arranged for him to "commit suicide" in his jail cell. As Wayne pointed out earlier, the novel (with more detail, as usual) had a passage in which two NYC detectives called on Kay to question her about Michael's whereabouts because he was under suspicion. The Bocchicchio Family solved that problem--another key detail missing from the film.
SC's note about this being the only time Vito ever uttered a threat is very important: it's a constant theme in the book, and it's what separates Vito (and, in his own way, Michael) from the other gangsters.