It seems like you should be able to climb up the tower behind Vieri de' Pazzi and his two bodyguards without being detected. Alas, the game forces Ezio to the Exposed state as soon as he reaches the top.
The goal for this video was to quickly get behind Vieri and to not fight any of his men - just him. I experimented with the new Taunting move. Notice how none of the other men attack as Ezio repeatedly Taunts Vieri.
After I had my fun Taunting Vieri de' Pazzi, I finished him off with a Hidden Blade counter.
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The developers try to force an Exposed assassination of Vieri de' Pazzi. If you climb up his tower, you'll become Exposed when you reach the top, regardless of whether they can see you.