Neither the novel nor the movie ever stated, claimed, ot otherwise explained how Vito became a member of Honored Society. He sort of just assumed a membership position after he killed Fanucci. That doesn't square with my understanding of how that Society did business then or now. Your thoughts!

Your point's well taken, Olivant. But Puzo wasn't inaccurate in not providing Vito with a formal "Mafia card":
The NY Five Families weren't formally established until 1931. Before that, there may have been dozens of little Mafia organizations in individual neighborhoods, operating small-scale. Following his victory in the Castellemmarese War of 1930-31, Salvatore Maranzano consolidated about 600 NY-area Mafiosi into five organizations, each with a capo reporting to him. He then declared himself capo di tutti capi and demanded a monthly tribute from each Mafioso. This didn't sit too well with the younger guys, who appealed to Charlie Luciano (one of the five capos reporting to Maranzano) to do something. Luciano contracted with Meyer Lansky to whack Maranzano. Afterward, the five capos became five Dons, each with his own independent family, joined loosely in the Commission, which also included four out of town Dons. Luciano was the informal chairman of the Commission, but he was smart enough not to declare himself capo di tutti capi. He didn't have to: he was considered first among equals.
Getting back to Vito: His initiation into organized crime started around 1919, with his murder of neighborhood boss Fanucci. As the novel points out, he took over Fanucci's rackets and expanded them by accumulating political power. By the early Thirties (when Puzo sets Vito's battle with "Maranzano,") he was probably a de facto Mafia Don. That's why there's such frequent speculation on these boards as to which Don he most resembled.

Regarding Maranzano, did he still have his own family/business while he was capo di capi tutti? Or he had to give up his family to other Don?
In The Sopranos when you became boss so you appointed your trusted man to run your former business, right? And that trusted man automatically promoted to capo.

Maranzano put Angelo Caruso in charge of his family after he became capo di tutti capi. After Maranzano was assassinated, Joe Bonanno took over.

Two things. One, Charlie Luciano took over what was formerly Maranzano's family Joe Bonanno was one od hrt eoriginal Dons with his own family that Maranzano put in place.

Vito would have to have been initiated into the Mafia in some manner. While the families may not have been formally established back then, acquiring membership in the Mafia was not just a personal decision or else all kinds of people would have been part of it.

THIS IS A WORK OF FICTION-IT NEVER REALLY HAPPENED.
That aside, Fannucci was never in the mafia either, he only claimed to be. When Vito whacked him, he gained the respect of the neighborhood. With thier respect and his "reasonableness" he was able to rise in the neighborhood, then then became a force outside as well.