User blog comment:B214/Question and Answers 6/@comment-24098614-20170712211422/@comment-10330822-20170718090401

They only fought once, and that was non-conclusive. My guess is it depends on how strong Issei's DxD for is compared to Vali's DxD L form, there is a high likelihood that Issei is stronger, considering the "source" of his power is superior to Vali's, but we won't really know until they fight.