Question: why vaan can see rasler?
Answer: Going into Raithwall's Tomb, Vaan is a vengeful spirit, much like Ashe. The Occuria know this, so when they show Rasler to Ashe, they are showing Reks to Vaan (we see Rasler, because it turns out to be more important to the story). They see the potential to make Vaan the next Dynast King.