This has always been one of those things for me as far as VC goes.
It was Diaz. The whole Sonny and the drugs thing is a total red herring.
There's far too much involved for Sonny to have orchestrated just to give Tommy something to do in Vice.
The Forelli's are crapping themselves because their most notorious killer is about to be released from a prison sentence they thought would be endless. He's either going to screw up their relations with the other organizations by his being on the street again or he's going to come after the family, blaming them for his incarceration.
The solution? Let's set him up a welcome home drug deal in Florida and sling him $3mil just to let him know there's no ill feeling. Hell, maybe he'll build up an organization - one we can take down at a later stage. They're pretty happy with this, as you can see in the first cutscene.
The rest takes place behind the scenes.
Forelli sends Ken Rosenberg to arrange the deal with Colonel Cortez and the Vance Brothers. Cortez is like a neutral broker, if you will.
Gonzales, being Cortez's right hand man, has the inside track on the deal and squeals to Diaz.
Diaz employs contract killer and communications expert Leo Teal to gather the time, date and location of the deal. Assume he bugs Rosenberg's car, because the Vance brothers fly in by chopper.
Teal jumps the deal with the Streetwannabes Guy and some other unknown goon. Vic Vance and the two Forelli goons are killed. Teal's crew recover the merchandise for Diaz.
The aftermath? Vercetti beats Teal to death and silences their only real lead.
He takes Teal's phone. The unknown phone call Vercetti gets afterward is from the streetwannabes guy, who is promptly spooked. He steals from "Diaz's merchandise" and goes into hiding, hence The Chase. Notice how jittery he is about his location?
Cortez has Vercetti silence Gonzales. Gonzales is protected in that mission by two Diaz goons. Cortez distances himself further from suspicion, all while having Tommy run around town thinking Cortez knows more than he actually does.
Diaz has Vercetti track down the streetwannabes guy in The Chase and recover the money in Phnom Penh '86. And that's the last you hear of it.
Mindf*ck or what?
If anyone other than Diaz messed up the deal, then my money is on Cortez arranging the set up.