I don't believe that Vito thought he was sending Luca on a suicide mission--even though it was absurd to think Luca could convince Tat and Sol that he was considering leaving the Corleones. As has been said a thousand times, the Don was slippin.'

Rocco's mission was definitely one-way. After humiliating Tom for saying it was impossible to kill Roth, and inviting him to leave the family if he didn't go along with him in the things he had to do, Michael turned to Rocco. In effect, he gave Rocco no choice. Since Neri had been maneuvering to get ahead of Rocco and Tom, Rocco may have viewed the mission as a last chance to get back in to Michael's good graces. If he said no, he'd be out--perhaps killed sooner rather than later. The brute-force was that Neri was Michael's choice while Rocco had been Clemenza's man. Rocco was expendable, Neri wasn't.

Michael definitely took a big risk sending Rocco on that mission because Rocco was identified with him. But he was determined--obsessed--to nail Roth. Rocco dead on the spot was less a threat to Michael than Rocco alive and in police custody. And Michael knew it, which is why he said, "I want it [Roth's plane] met," instead of suggesting a plan that was less risky to Rocco.