Not in the sense that he stood by Vito's side on a daily basis.

Luca was more like Vito's pitbull. Vito may summon him for that 'special' job. And Luca was probably very protective over Vito in the sense that if anyone attempted to or did bring harm to Vito, Luca was a one man hit squad.

We learn about this in the novel, and it is quite obvious in the movie as Sollozo made sure that Luca was out of the way when they attempted to kill Vito.

Luca was a 'special' person in the Corleone family. He really was not under any regime's command. He was not given orders through a chain of command either. He basically reported directly to the Don.

Don Cardi