In one of the corners near the vista there's a bunch of gravestones that you can use to get up onto the roof of one of the tombs. From there you go around along the wall a bit to a gap between the vista with a tree in between. You need a swiftness boon to make the jump, but that's easy to get from the priests at the top of the walls.
You dont need swiftness. A lot of vistas and puzzles are easier with swiftness, but none actually need it, its just a much harder jump if you dont have swiftness. I find my thiefs natural 25% boost is enough.
This series is the Inception of games. Everyone is inside each other, and it only goes deeper... - DrunkBeardGuy on Kingdom Hearts Proud 3DS Ambassador