Ah. So the new vendor will only show up when I have 10 ACTIVE insights (as shown in the upper right cover of the screen)? Can I just use the stock insights to boost up the number? Or I have to get insights via the "legit" way? I have 8 active insight now and a bunch in my inventory. I actually don't know how I gained those 8 insights...
They have to be active.
You get 1 insight for finding a boss and 2 (or 3?) for killing a boss. You can also get 1 for killing an invader or for killing a boss as a guest.