User blog comment:B214/Volume 21 info & spoilers/@comment-118.172.205.199-20160702052735/@comment-24.113.252.213-20160702063027

Something similar happened in Vol. 20. Issei was beaten so badly, by Rizevim and Belial, that he could barely stand. Yet he was still able to not only stand, but also use his sacred gear and keep fighting, all because of his "will to fight."

Remember, Sacred Gears are powered by the users will and emotions. They are the catalysts for the realization of the users ambitions and desires. So, as long as there Vali or Issei still have the "burning" desire to stand and fight, that "burning" desire will serve as energy to keep using their Sacred Gears.

If you're wondering how/why Vali met this condition in his fight with Azi Dahaka, recall this:

Azi put Vali in an illusionary world where all of Vali's deepest desires were real. He was living a normal life with his mother and two younger siblings. Many heroes Azi faced in the past had their wills broken by this illusion, but Vali was able to resist and come back with renewed, and much stronger resolve. He vowed to forever accept who, and what he was, and use his power to protect his family. Seeing Vali's determination, Albion also decided to accept himself (his poisonous power that others feared him for), gaining his own "burning" desire to become stronger and fight alongside Vali.

With both of them having such strong emotions (and Ophis' help), Vali was able to enter DxD L mode, spam a lot of powerful abilities, and stay in that mode for an extended amount of time without exhausting himself.

I apologize for how long this is, but I hope it answeres your question.