I think the Tattaglia/Barzini axis didn't count on Vito returning to active duty - I think they figured he was more or less out of the picture, and by getting rid of Sonny they would be in effect decapitating the Corleone structure. Vito getting up out of his sickbed had to be a big surprise to them.

They probably figured that getting rid of Sonny would set off an intrafamily battle in the Corleone organization between the regimes of Tessio and Clemenza, thus neutralizing both of them.