Can I prevent my opponent from looking at cards with Vizier of the Menagerie while I'm Mindslaving them?

Question

Vizier of the Menagerie says "You may look at the top card of your library," and Mindslaver says you "make all decisions for the player."

What I'm wondering is can the player who's being controlled see the top card of their library as their turn is taken for them? In other words, if I'm Mindslavering someone and I get to make all decisions for them, then I get to decide when they look at the top card of their library, so I can decide that they don't get to look at it.

I'm not sure about this, but based on all the Gatherer rules, I would say that the controlled player and the controlling player can both look at the card because the card becomes part of the "known information" to the player who controls the Vizier (so they can always know it) and since they always know it, the Mindslaver controller also knows it.

Answer 1

Yes, the Mindslaver's controller may prevent the controlled player from taking that specific action.

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Mindslaver's rulings say:

You get to make every decision the other player would have made during that turn. You can’t make any illegal decisions or illegal choices — you can’t do anything that player couldn’t do. You can spend mana in the player’s mana pool only on that player’s spells and abilities. The mana in your mana pool can be spent only on your spells and abilities.

Since looking at the top card of your library is an action you may take while controlling Vizier of the Menagerie, the player controlling the turn may simply choose not to take this action.