Obviously he was not planted anywhere. I think when Zassa was told by Michael that his dispute with Vincent was his business, Zassa thought maybe he could the failed hit on Vincent.

Even after the hit Michael is mad at Vincent for killing the two goons because it is causing trouble in the commission.

Also, later on in Atlantic City when they are passing the money arou0nd to everyone Vincent does a a very "un Corleone" like thing when he passes Zassa and says "Nothing in here for you."