Good point, Oli, but I think the combination of Sol's temperament and Barzini's ambition is why they hit Vito. For Sol, Vito's refusal to provide political and judicial protection was an insult and very harmful to his business, so he had the motive.

For Barz this was the perfect opportunity to knock off the Don of Dons, and basically replace him without anyone other than Sol knowing who was pulling the strings. As has been discussed elsewhere Barz was probably going to eliminate Sol as soon as he could in any case.