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.