Ok now I know some people think he was the underboss but I think he may have been a Capo and heres why if anyone knows better than you can explain it to me.

1.The book states the Corleones had never used that position.
2.He had his own crew

Thats really all I picked up but if other people have evidence that he was a Capo or otherwise please post here.

In the novel, just after the Corleones moved to Long Beach, a "boiler inspection" crew calls on the Don and makes a problem. The novel states that the Don gave the problem to Sonny because he was "considering making Santino his underboss." Now, it didn't specifically say that Sonny was formally titled "underboss." But he was, by default, Vito's successor. And he was acting boss after Vito was shot, and even after Vito came home from the hospital, and had authority over Clemenza and Tessio.
Sonny probably did have his own crew, but they may have been bodyguards (like the bunch that accompanied him on his visit to Lucy), or may have been detached from Clemenza's or Tessio's regimes. As a guy who was clearly on a rung above the two caporegimes, he could get what he wanted.

Yeah but it stated in the book that the Corleones had never used the underboss position I rember the part when the boiler inspectors came but I thought after the way he ended it Don Vito would have thought it as failing and also in the book he had his own Regime they called it the Santino Regime and that after he died in one part it said No one took over the Santino Regime and I think Don Vito even went as far as dissolving that crew I belive Willy Cicci was a part of it. (It showed him outside Lucy's house in the game but again thats the game and the game had him listed as Underboss)

No underboss and only two capos to handle 1,000 soldiers.

Also, I've always wondered under what formal(?) circumstances if any Vito was "made" and Fredo, Sonny, and, of course, Michael. It seems that Vito just kind of started acting like a Mafia Don and thus became one.

Neither the book nor the films ever give any indication that any of them were formally made. This follows real life: Not every (or even most) Mafia families have a formal "making" ceremony as described by Joe Valachi and others.

turnbull....i havent read the novel in a while but i thought it mentions a ceremony that is in the works for micheal.

I'm referring to Puzo's novel, not Winegardner's...uh...book. Winegardner has some kind of ceremony for Michael (my memory of that book is dimming--deliberately). Absurd, since Michael became acting head of the family immediately after his return from Sicily