It's possible that the reason Vito was hit was to FORCE Sonny to go in on the the deal instead of retaliating, which would be his (and Luca's) natural response (as Solozzo admits) and then there's the next war. By peacefully going in on the drugs deal even after Vito's death and the Family's loss of political strength, the Corleones are still in business but greatly weakened and Barzini is on top.

That is probably what Barzini/Tattaglia/Solozzo were hoping for...but of course did not count on Vito surviving those shots.

It makes sense that Tom would make that suggestion...after all, it wasn't until that second failed attempt at the hospital that even Michael knew they were going to come at Vito again.