I assume his plan B would have been (was) Carlo. Vito may not have trusted him entirely but Carlo certainly would have had enough access to kill Vito himself (although he seemed like too much of a coward to do so) or to, for lack of a better term, "open the drapes" for Sollozzo/Barzini's men.

Also, was it really necessary to kill Luca before the hit on the don? It didn't seem that Luca was always at the Don's side. For instance, he didn't expect to be invited to the wedding, even though there would be many strangers around the Mall that day. Plus, as pointed out above, he was easy to spot. Despite his reputation, I imagine he would have been susceptible to ambush.