The Wooden Vajrapani Mask is the hundredth treasure that can be found in Uncharted 2: Among Thieves.
The Wooden Vajrapani Mask is found in Chapter 25 - Broken Paradise. This will be the one-hundredth treasure you've encountered, though there's one more after this one. To acquire it, leave the chamber where the fray just occurred (and where you found the 99th treasure), going up the staircase straight across from the stairs you used to enter in the first place. This will ultimately lead you to another series of staircases. As you swing left to go up another staircase, look up and to the right, and you'll see this treasure shimmering on the wall. Shoot it down, walk up the stairs, and grab it. Easy.