Vizzini should have known both were poisoned.
Wes could have easily just killed Vizzini. He took down a giant. I'm sure he could have beaten Vizzini, a short fat man armed with a dagger, without a sweat. Why would Wes risk a 50-50 shot when he could easily just kill Vizzini and take Buttercup? He wouldn't. That would be stupid and illogical. He only did the battle of wits because he wanted to look like a suicide.
Vizzini knowing that should have also known that the battle of wits would be set up in a way in which he couldn't possibly win, because what idiot would choose the option that gives you a much smaller chance to win (a 50-50 "true" battle of wits) over the option that gives you a huge chance to win (Wes just killing Vizzini with his sword)?
Vizzini should have just bargained Buttercup for his own life or tried a surprise attack on Wes when he turned his back.