I agree Turnbull. Vito was nobody's fool, and the fact that a Don with his experience and insight missed the mark on Barz speaks volumes about him losing his edge.
He never respected Tattaglia as a true Don, and had to know that there was other involvement in his drug business.
The Luca mission aside,after the attempt on his life,and the murder of Santino, [Also car bomb on Michael's life] Vito had to know it couldn't just be Tatt.
He should have seen,in 10 ft high neon letters "BARZINI"
I find it almost beyond belief that it wasn't until the meeting that his eyes were finally opened.

Also, Tom's surprised response: "You mean Tattaglia?" strongly suggests that in all the time he was recuperating, he never discussed the issue or his suspicions with his Consiglieri.