At the meeting with the 5 families Vito is talking about what would happen if something happens to Michael when he returns to America. He says something along the lines of "if he should be found hanging in jail cell" or something like that. What I wondered was why does Vito mention jail. It's like as if he expected him to be going to jail as soon as he got back.

Possibly because the police would arrest him on suspicion of murdering McCluskey? There might not have been any actual evidence (even though he did it) but the novel has a part where two detectives talk to Kay about Michael's whereabouts because they supposedly are certain he's the murderer.

The quote is:

" All right -- and I have to make arrangements to bring him back here safely -- cleared of all these false charges. But I'm a superstitious man -- and if some unlucky accident should befall him -- if he should get shot in the head by a police officer -- or if he -- should hang himself in his jail cell -- or if he's struck by a bolt of lightning -- then I'm going to blame some of the people in this room. And that, I do not forgive".

Vito was referring to any possible attempt on Mike's life (made by Barzini, et. al) as Mike was being brought back to America. This is the closest thing to a threat ever made by Vito.

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.

When he said that "he was gonna blame some of the people in this room if anything happened to his son" was he giving a warning or threat that he was gonna get you if any of the Dons laid a finger on his youngest son?

That was a warning and a threat, I would say as he warned the Dons to not interfere with Michael coming home to the United States and he promised that he'll take action if any incident occurs with the death of Michael that he'll attack some of the people in the room.

Another threat Vito made is when he told the bandleader, "Either your brains or your signature will be on that contract".

Another threat Vito made is when he told the bandleader, "Either your brains or your signature will be on that contract".