Unfortunately, this is one instance where the movie glosses over Vito's plan.
I see Luca as more of a loyal meathead/hitman than a cunning spy.
The opposing family would have surely known about Luca, and his strong affiliation with the Corleone's. Any word otherwise would be laughable, given the circumstances.
Brasi was sent on a mission that required subtlety and cunning...not something you'd use a "meathead" for.
Luca Brasi had made contact with the forces of Sollozzo several months before. He had done so on the orders of Don Corleone himself. He had done so by frequenting the nightclubs controlled by the Tattaglia Family and by taking up with one of their top call girls. In bed with this call girl he grumbled about how he was held down in the Corleone Family, how his worth was not recognized. After a week of this affair with the call girl, Luca was approached by Bruno Tattaglia, manager of the nightclub.
The first meeting was all above-board, Tattaglia offering him a job to work in the Family business as enforcer. The flirtation went on for nearly a month. Luca played his role of man infatuated with a young beautiful girl, Bruno Tattaglia the role of a businessman trying to recruit an able executive from a rival. At one such meeting, Luca pretended to be swayed, then said, “But one thing must be understood. I will never go against the Godfather. Don Corleone is a man I respect. I understand that he must put his sons before me in the Family business.”
The general idea had been to lead the Tattaglias to believe that he knew about the lucrative narcotics operation and that he wanted a piece of it freelance. In that fashion he might hear something about Sollozzo’s plans if the Turk had any, or whether he was getting ready to step on the toes of Don Corleone. After waiting for two months with nothing else happening, Luca reported to the Don that obviously Sollozzo was taking his defeat graciously. The Don had told him to keep trying but merely as a sideline, not to press it.
The Godfather - Mario Puzo
So Luca wasn't "infiltrating the ranks" per se, just gathering intelligence by association with the Tattaglias.
None of this fits with your interpretation of Brasi but, again as I mentioned, the impression given by the movie leaves a lot of Brasi's story on the cutting room floor.