User blog comment:Jopjopjop/Volume 20 Spoilers/@comment-26285797-20150721203748/@comment-3965848-20150721204250

Well including Issei's new form (that is if he can still use it) Vali would be unable to half Issei's power since it would still be infinity and Vali would be unable to coupe with the absorbed power.

So Issei would win.