You're right, Lilo. It is hard to believe that Vito thought the plan with Luca would have worked. (I'm not sure Luca thought at all.) In his defense, in the book the plan is a little more drawn out than simply having Luca walk up to the Tattaglias and tell them he wants to join them.

The parallel Michael/Vincent plan had a better chance of succeeding, if only because Vincent wasn't known to be as being excessively loyal to Michael, and he has a motive for dissatisfaction (Mary).

This brings up another problem with Rocco's assassination of Roth...

Michael had just narrowly escaped prosecution on organized crime charges. Why would he send someone publicly identified as one of his top men on a suicide mission to kill another mobster? I know it was an important job and all, but, if only to preserve whatever veneer of legitimacy he had left, it would have made more sense to use a Busetta type - someone with no apparent connection to the Family.