It seems most likely that he was just carrying a picnic.
Consider: Vizzini considered it "inconceivable" that the man in black could defeat Inigo:
"See?" Fezzik pointed then. Far down, at the very bottom of the
mountain path, the man in black could be seen running. "Inigo is
beaten."
"Inconceivable!" exploded the Sicilian.
The Princess Bride
He also considered it "inconceivable" that (earlier), the man in black could catch up with them:
"I can feel him," Fezzik said. "His body weight on the rope."
"He’ll never catch up!" the Sicilian cried. "Inconceivable!"
The Princess Bride
All of this would have to happen before the man in black could engage in a battle of wits with him.
Why, with all this and his exceeding arrogance, would Vizzini bother to make elaborate preparations for an event that he had never conceived could occur?
It’s made pretty clear that his foresight is far inferior to what he believes it to be.
Besides, Vizzini spends a lot of time threatening to kill Buttercup in order to stop the man in black from attacking him. Why bother to threaten her if he had sufficient foresight to know that he could entice the man in black into a game of wits.
Don’t forget that Vizzini has not only wine but also cheese and apples.
Vizzini was waiting for him.
Indeed, he had set out a little picnic spread. From the knapsack that
he always carried, he had taken a small handkerchief and on it he had
placed two wine goblets. In the center was a small leather wine holder
and, beside it, some cheese and some apples. The spot could not have
been lovelier: a high point of the mountain path with a splendid view
all the way back to Florin Channel. Buttercup lay helpless beside the
picnic, gagged and tied and blindfolded. Vizzini held his long knife
against her white throat.
The Princess Bride
This is also the case in the movie:

Vizzini’s going on a bit of a journey. He needs to eat. Why not bring some food and wine?
That still does not explain why he laid out all the food. Most likely, though, he was sufficiently confident that Fezzik would defeat the man in black (surely it was inconceivable that he could lose) that he believed it safe to have lunch, and either intended to eat with Fezzik or would have forced Buttercup to join him (thus the two goblets). Once he saw the man in black coming, he held his knife to Buttercup for protection.