He told him. Vizzini told that meddlesome man in black to never go up against a Sicilian when death is on the line. The man in black smirked delightedly when Vizzini slumped to the ground after drinking that iocane powder. It had been clear that the man in black knew how to fight once he passed through both the Spaniard and the giant unscathed. Unfortunately, Vizzini wasn't built for combat.
What he lacked in strength, he more than made up for with his intellectual fortitude. When death surrounds you, it only makes sense to limit the things which can kill you. Throughout his travels, Vizzini had learned of iocane powder and he had learned how to adapt to it. Luckily the man in black did not check for a pulse. He was too busy dragging the Princess away to notice that Vizzini was still breathing, however slightly.
When the man in black left. Vizzini remained where he was with a chorus of thoughts, enjoying the acoustics of his mind. He finally decided to stay put and wait. He knew that the Prince would arrive sooner or later. Even though Humperdinck himself hired Vizzini to abduct Buttercup, he couldn't let on that he knew anything.
When Vizzini heard the distant galloping of hooves, he played dead once more. Soon the sounds of horses and clanking metal surrounded him, and he could hear the Prince himself analyzing the situation like the trained hunter that he was. Humperdinck reached down to feel Vizzini's pulse and the Sicilian stiffened. He was delighted when Humperdink pronounced him dead and suggested that they ride off in search of the mysterious man in black.
Vizzini was fetched later by members of Prince Humperdinck's personal guard. He was brought back to the castle and given a large meal, for laying out in the grass for hours had made him quite hungry. As he finished his meal, Humperdink joined him to inquire as to the events that had transpired.
"Everything was going as planned." the Sicilian explained. "We had reached the Cliffs of Insanity and the Princess would soon be dead. Before we scaled the cliffs, one of my men spotted a figure following us. A pirate from the looks of him. I wouldn't be surprised if it was the Dread Pirate Roberts himself the way he scaled that cliff-side. Anywho, I sent my men to distract him so that I could escape with the Princess. Unfortunately, he caught up with me, and I had no choice but to make him think he had beaten me."
The Prince looked on smugly as Vizzini regaled the events. "I know of the man of whom you speak. His name is Westley, a simple farm boy who had old feelings for my beloved Princess." Of course calling her beloved was merely a formality at this point, as both men wanted her dead. "He has been taken care of, and Buttercup is safely back in her chambers."
"Should I carry on the job as before?" Vizzini asked, eager to hear approval. Unfortunately for him, he did not.
"No, I think not. It is clear now that the wedding must go on. My people will be much more devastated to hear that their Queen was murdered instead of their Princess, and on her wedding night no less." The Prince seemed so confident in his plans. He had thought every detail through, much as Vizzini had always done.
"I assume I have my part in this plan?" Vizzini asked. He would hate to be kept out of the action.
"Indeed you do." Humperdinck confirmed. "A rather large one at that."
On the day of the wedding, the plan was in motion. The Brute Squad was patrolling the streets and Humperdinck had used the machine to bring Westley an inch from death, enough so that when he was found, he would be taken to Miracle Max for help.
Vizzini was hiding in the back of Miracle Max's, along with Max's wife Valerie. Humperdinck had given Vizzini a vast amount of gold, enough to bribe Max to give Westley and his companions a tainted medication. Once taken, the pill would bring Westley back to health for a few hours, before reverting him back to a helpless state. This way, Humperdinck could frame Westley for Buttercup's death and the people would praise him for killing an assassin.
It was important for Valerie to storm out at the right time to cause a fuss. In order for Westley's companions to trust Miracle Max enough to give Westley the pill, they had to believe that Max and Valerie hated Humperdinck as much as Westley did.
Finally, at the time of the wedding, Westley, Fezzik, and Inigo Montoya stormed the gate. The Prince sent Count Rugen to stall them long enough to get Buttercup to the honeymoon suite. As Inigo combated Count Rugen, Vizzini waited for one of them to win. He almost wanted the Spaniard to win. He had proven a valuable-enough asset and the Sicilian didn't want to see him die. That being said, as soon as Inigo killed Count Rugen and sheathed his sword, Vizzini knocked a bottle over his head, knocking him out cold.
Vizzini, with the help of a few guards, carried Inigo's unconscious body to the honeymoon suite. Westley stood there, holding a sword up to Humperdinck. Without warning, Westley stumbled backwards and collapsed on the bed. Humperdinck picked up his sword and the guards threw Inigo's body next to Westley's. The guards left the room and Humperdinck complimented Vizzini on his actions.
Buttercup, horror-struck next to the bed, saw her chance and grabbed Inigo's ornate blade from its sheathe and struck it into Humperdinck. Caught off guard, Humperdinck slumped to the floor as Buttercup pointed the tip at Vizzini's chest.
"Inconceivable!" The Sicilian exclaimed. Fezzik's voice called from the window, and Buttercup told him to come up and help. Together, they got Vizzini to tell Max to make the correct pill, so that Westley could fully recover. They then imprisoned Vizzini, and lived the rest of their lives happily ever after.