Vanya vs Darklight?

[Diskordea]

So my FC plans to farm Garuda EX. I asked when we would be doing that and was then asked if I have the full Darklight set yet. I said no then was told I was not ready then and was given no answer to my "why Darklight over Vanya" question. I have full level 70 accessories and the Vanya set with Materia added. Don't quite understand why having the Darklight set is such a big deal if I already have item level 70 stuff.