User blog comment:B214/Question and Answers 6/@comment-99.235.66.221-20161130072057/@comment-29963364-20161214031107

You do realize that the reason Vali survived being poisoned by Samael for as long as he did was because of his "vast amount of Demonic Power", right?

The case of "who survived longer after being poisoned" doesn't have anything to do with "who is physically stronger."

Both Issei and Vali are about equal in physical strength and durability (Issei might be a bit above, since he is stronger than Vali in terms of Attack Power & Direct Speed), but Issei has very little Demonic Power, while Vali has a lot. So it stands to reason that Vali could resist the poison for longer.

I'll bet that if Vali didn't have his "vast amount of Demonic Power", he would've died the same as Issei.