As far as I'm aware, you need to do both the Zaith'isk and the Dream Guardian scene inside the Astral Prism (And subsequently let Lae'zel into your mind post interaction)
You might get away with skipping the Astral Prism as Lae'zel isn't too concerned about it until Vlaakith shows up (At which point you have to go along with Vlaakith or else Lae'zel will turn hostile). Though I'm not sure how this would impact things later in Act 3.
