Choosing vlaakith's side makes keeping lae'zel impossible, period. Enter the creche in act 1, and you'll face the critical choice that defines her loyalty. Following vlaakith's orders to eliminate the guardian marks you as opposition to lae'zel's eventual alliance with orpheus, forcing her to leave or turn hostile unless you pass extremely difficult persuasion checks.

Want to keep lae'zel? Side with the reformist githyanki, supporting orpheus and voss instead. This unlocks the silver sword through kith'rak voss, and honestly, it's the smoother path since lae'zel naturally discovers vlaakith's deceptions about the astral prism anyway. The game pushes you toward this choice, making the traditional githyanki path deliberately difficult to maintain without losing her.