Vito's policy was to give orders to one person at a time, so that if anyone turned rat, prosecutors wouldn't have a corroborating witness--someone who'd testify that he'd seen or heard the same things that the rat testified to. We saw this when he told Tom to tell Clemenza to beat up the punks who ruined Bonasera's daughter, and when he gave Luca the order to infiltrate the Tattaglias.

So, why did he have Sonny, Fredo, Tom, Clemenza and Tessio present when he met with Solozzo? You could argue that he wasn't giving a criminal order--he was actually turniing down a criminal partnership. Why would he need witnesses for that? Your views?

I imagine it was so that everyone would know his opinion on the matter, and that it would be pointless for Solozzo to try and get around Vito's "no" by approaching any of the others individually. Something along the lines of presenting a united front. But then, Sonny ruined it all.

Agreed and he wanted the entire family and other families to know that the Corleone's do not deal in drugs as passed down from the Godfather's own mouth.

Well, in the book it's just Tom and Sonny.

In his early notes, Coppola wrote that he sort of wants to bind Vito and the audience here. Sollozzo is supposed to be scary, and Vito the protector. So maybe the more guys, the more safe the audience feels by identifying with the Corleones.

I think Vito understood the greed that drug profits stimulate (per Sonny's gaffe). So, in addition to signaling Sol that it would be pointless to approach others under him, Vito was also assuring that Clem, Tess, Sonny and Fredo wouldn't be tempted to approach Sol on their own (the novel says Sonny was looking for action on his own that would get him out from under Vito's thumb).

Sonny's display of greed no doubt emboldened Sol to take his shot at Vito. And, he probably told Tatt and maybe the other Dons that Sonny's greed meant that he'd accept a deal, and that there'd be no war. Ironically, Sonny's gaffe in front of all those witnesses guaranteed that he'd never accept anything from Sol except his dead body. If he'd made a deal with Sol, Clem and Tess and maybe Fredo would think he was none too sorry to see his father--and his father's objection to drugs--out of his way. It would have wrecked Sonny's credibility as head of the family.

I think it was all FFC's doing. He staged that scene to illustrate all of the Corleone power brokers in on a decision just like he staged the post-Vito assassination when all of those power brokers (including Michael) were in Vito's office trying to figure out what to do next.

If I were Vito I would want all of them there so that Sollozzo would understand the Corleone unity. I've never believed that Sonny's gaffe was because he was somehow maneuvering for his own operation. Sonny's question was a practical one and I think that Puzo's and FFC's presentation of it is a hole in the plot. Vito's no was enough reason for Sollozzo to go after Vito.

Solozzo came with flattery and generosity for Vito's politicians in his pocket
He was always going to be killed, once he said no, with or without Sonny's gaffe

No doubt he was evil, ruthless, murderous and I reckon his no was keeping in line with him, cleverly being portrayed as the nice, family man that he wouldn't touch the dirty drugs business in spite of projected profits of millions

Sollozzo must have thought there was a chance of the drugs business “criminal partnership” with Vito however sure thing “He was always going to be killed, once he said no, with or without Sonny's gaffe”

Sollozzo was slippin' expecting Sonny to "accept a deal” - believing Vito was dead [first attempt] - as if....after killing his father for refusing the same deal