User blog comment:B214/Volume 22 spoilers/@comment-28087053-20161030231922/@comment-10330822-20161112225211

Except we don't know who is more powerful after the months in-between that fight and the tournament. For all we know, Issei could be on a whole other level now.

Besides, in the long run Issei should be stronger. His DxD G form uses the power of Ophis's Infinity, and he just has a superior body. Vali's Lucifer powers should only be able to take him so far, since its already pretty much confirmed Great Red is dimensions away in terms of power when it comes to the original Lucifer. Should Issei learn how to handle the full power of DxD G, he would far outclass Vali.