There's the door near the Demon of Song which only open if you're hollow. Then there's also Navalan where you can only start his quests if you talk to him while hollow.
Does anyone know why his soul is just sitting on a chair in some random cave he'd be too big to fit into anyways? Is there any explanation behind this that I've missed?
No, not really. Maybe they wanted to remove an incentive to kill Vendrick? In my own humble opinion, killing Vendrick should give the player his crown and soul. The ring and clothes should be on the floor. Simple and logical.
Vendrik used the power he took from the giants to remove his soul from himself, because Neshandra wanted to corrupt it. And he put it in Amone because the singing ladys suppress creatures of the Dark.
That's pretty clear. I really am not sure what more you would want from a description. It's kind of hard to describe the door if you don't know what you're looking for.
So... To get all 3 Boss items I have to go through 2 more NG Cycles? Oh great. Sad I can't just respawn Vendrick and pick the soul again. Weird decision imo.