I don't think Tom was even informed about Vito's plan to send Luca to find out what was underneath Sollozzo's fingernails, so you can't blame that one on him.
Agreed, dt: Luca took his orders directly from Vito.
I was thinking about this too, but I came to a different conclusion. I think, Vito must have informed at least Tom about this move. If some informers had seen Luca hanging around in the Tattaglia bars and frequently talking to Bruno, they would have told Tom anyway.
