After Michael's visit to the hospital, licensed, armed private eyes guarded Vito. Plus, after Mac was shot with Sol, the Police Commissioner probably thought it was politically advisable to have cops on hand to protect Vito, the better to prevent any further attempts on Vito that could prove embarrassing to the NYPD.

I'm guessing that, since Vito was totally incapacitated, and guarded round the clock, the other families would leave him alone and target Sonny, who was running the Corleone end of the war and was far more dangerous to them at that point.