The second was sending Luca on that ridiculous mission to Tattaglia with the idiotic story that he was “not too happy with our family.” Tatt and Sol saw right through it. Vito handed them his “most valued friend” on a silver platter.
I don't think Vito really saw Luca as his most valued friend - he didn't fit very well into the Corleone future. It's Vito's insight that Luca was essentially suicidal, and I think Vito was long resigned to the likelihood that the day would come when Luca did not return.
Again, I think the "Barzini all along" refers to a time when Barzini was manipulating both Sol and Tat, but hadn't yet aligned with them, even secretly.
