OK so remember that super-fun boss battle where you destroyed the black heart of the black garden in vanilla? Me neither, but I found a huge plot hole in that section. If the vex wanted to protect it so much, then why didn't they prepare for the guardian to arrive since they know of their power? Let me explain. The vex are controlled by one huge, shared mind right? Then why didn't the heart just bring all the 3 statues to life and brought like 20 ultra minotaurs and 3 legions of harpies to defend itself against the guardian's attacks? Since they are all shared by one mind, wouldn't the attacked vex just before the heart room have communicated with the other vex by their shared mind tactics to warn them to not underestimate the guardian? Plus, when the guardian arrived, the vex seemed suprised to see one and slowly turned around one by one because they felt it or something. I don't know but any theories guys?
Because Destiny has a 10 year plan. The Vex know that their prolonged existence is dependent on the Guardian succeeding on their journey. Defeat the Guardian, then the Vex will be wiped from all existence. This is all part of the plan.