Though the novel gives us more detail than the movie on Vito's rise, neither explains how Vito got to be Numero Uno in the ruthless, competitive NYC Mafia arena and how he attained so much influence beyond NYC. What personal strengths and behaviors do you think made Vito the top guy?

For my two cents worth, In no particular order,a couple things that I think made Vito the leader that he was :

His reputation among the Families for being a man who preferred compromise over violence.
In the Novel,if Vito said that if a man would not listen to reason, it was generally understood by others that an unpleasant resolution might be the next option.

He had probably the widest political influence of all the Families.
He formed and maintained these relationships by staying with the rackets that wouldn't embarrass his Family or the reputations of the officials on his payroll,(gambling,unions,etc) as opposed to that pimp.Tattaglia.

All of the other old school mobsters respected Vito as a man who's word was iron clad,and he was not a man to make frivolous promises to.

He rewarded loyalty,kept a low profile,and in general tried to build lasting relationships with both underworld and upperworld people.

I'm sure there's other factors,but these are the first few that came into my head.

He also wasn't averse to a bit of brutality, though of course committed by someone other than himself.
During the wars that eventually put him on top, his rival sent word to Al Capone asking for some gunmen.

From the novel:

"Maranzano sent a call to Capone for his two best gunmen to
come to New York to eliminate the upstart. The Corleone
Family had friends and intelligence in Chicago who relayed the
news that the two gunmen were arriving by train. Vito Corleone
dispatched Luca Brasi to take care of them with instructions
that would liberate the strange man’s most savage instincts."

The novel goes into some detail about what happened next; brutality is a very apt word.

I wish Luca's storyline in the the Novel would have made it on screen. In addition to the incident described by lucab19,there was another horrifying one involving a midwife and a newborn infant. Luca was a sadistic, demonic man,and any sympathy for the manner in which he died would've given way to happiness at seeing him,in some small manner,suffer for his crimes.

He had Puzo and Coppola on his side.

II'd add a few more attributes:

He wasn't greedy. He shared with other Dons, always ready to make an accommodation. He knew that he could have made huge money with drugs, but he turned down Sol's drugs offer and stuck with gambling and unions, which were safer and slower, and easier for his friends in politics and the law to accept and protect.. A corollary:

He was slow to anger and to exact vengeance: As he said in the novel, "Revenge is a dish best eaten cold." He waited until Michael was safely back from Sicily and firmly established as his successor; didn't pull the triggers in his lifetime. Michael also waited before avenging Sonny and Appolonia, and whacking the other Dons.

The movie and the novel show us many examples of how greed, and lust for revenge, make strong men weak and stupid. Vito and Michael were neither.

He was careful. The novel tells us he never uttered a threat, thereby never giving his enemies advance notice of what he was planning. He also gave orders to subordinates one at a time, or through his consigliere, so there'd be no corroborating witnesses in case someone under him turned rat. We saw examples in GF when, alone with Tom, he tells him to give the job of avenging Bonasera's daughter to Clemenza, and when, alone with Luca, he tells him to get close to Tatttaglia. "Women and children can afford to be careless, but not men."
He looked beyond New York: His investment in Moe Greene's hotel was the basis of Michael's move to Nevada and "legitimacy." He also controlled unions in the movie industry that gave him influence in Hollywood: getting Johnny Fontaine the part in the movie that resurrected his career, and helping him (according to the novel) to win an Oscar.

Fanucci was stupid, Vito was lucky

My take, it was perfect times and opportunity / need -
1. Vito having lost his Grocery store job to tyrant Fanucci's nephew
2. Little Italy - their own Justice League

Vito killing Fanucci, earned Vito his formidable reputation
Fanucci's death created a vacuum and Vito stepped in, offering - always with a caveat - friendship and taking care of / settling all Little Italy business, dispensing his 'justice'
and everyone lived happily for a hundred years! as long as they were Vito's friend and no one crossed Vito

It seems to me -

• “Vito got to be Numero Uno” because during Vito's rise to the top guy, there was little to no competition?
• No other contenders? to challenge Vito
• perhaps “NYC Mafia arena” was not quite ruthless, competitive nor Mafia established! when Vito started out
• then Luca Brasi came along Vito's one man killing machine – Vito was invulnerable

Greed
If Vito's friends would have gone along, wouldn't Vito have been greedy for the “huge money” even if drugs was a dirty business?

Money can be a weapon even more so in their business where every Don is always greedy for money, more money indeed – apart from the money itself, the richer Dons would wield more clout....power, security and able to buy more cops, judges and politician friends

He was slow to anger and to exact vengeance:
Sure thing Vito “didn't pull the triggers in his lifetime” thus leaving a murderous legacy for “I never wanted this for you, son” Michael after - always the nice guy even when detrimental to Michael - Vito's death, to avenge Moe Greene “Sonny and Appolonia, and whacking the other Dons” and condemning Michael to Lifelong misery Thanks! Pop

He was careful
Vito was “careful” until he was careless
after Vito's no to Sollozzo, complacency and slippin' brought him undone and messed up Michael's life

He looked beyond New York:
Sonny's “investment in Moe Greene's hotel” formed “the basis of Michael's move to Nevada and "legitimacy."

You are right “He [Vito] also controlled unions in the movie industry that gave him influence in Hollywood:”
Vito didn't confine himself to just his own backyard and his nefarious power spread it's wings "beyond NYC" including Khartoum's stables

My take, for what it is worth! Extracts:

Lou_Para among others -

• Vito may have “preferred compromise over violence” but never shied away from violence to get his own way
• Vito's “unpleasant resolution options” "if a man would not listen to [his] reason" - Vito actually enjoyed the power and fear that he wielded - comply or die
• Vito 'justifies' his friends in politics rule his “(gambling,unions,etc)” business as a -- harmless vice [not for the victims]
• sure thing Vito had the foresight to cultivate “the widest political influence” - Mobster carrying those politicians in his pocket, like so many nickels and dimes

lucab19 “He [Vito] also wasn't averse to a bit of brutality” - a bit?! - "though of course committed by someone other than himself" among others -
1. Murder, extortion, threat of lethal violence, comply or die
2. the way Vito himself shot Fanucci, cut open Don Ciccio....
3. band leader's brains or his signature on Vito's Godson, Fontane's release contract
4. should anything happen to Michael on his return from Sicily -

Surely you're familiar with the great Aussie tradition of understatement?

True Lana from what was seen in the movie, Barzini was the only contender, younger, ambitious and impatient but was no match for Vito

It was the right time and right place, perfect times and opportunity / need - killing tyrant Fanucci, earned Vito his formidable reputation Murder, extortion, threat of lethal violence, comply or die "for justice, we must go to Don Corleone."

He was an astute observer.

You are good!

• Fanucci extortion - Vito the budding Don

reversal of the fish tank scene
Tessio agrees and Salute with Vito, Clemenza sulks, just sits there then toasts

Clemenza was the Boss before We have to pay him $200 each -- since he's so tough