User blog comment:Trihexa/Top 10 Most Powerful Beings In The World (Unofficial)/@comment-112.198.72.30-20170302202010/@comment-189.231.181.143-20170312040034

Because this is Vali's top ten, the ones he sees as strongest and wants to defeat.

Issei in a future might be one the Vali would like to fighyt and that is why he isnt in the list so as his own dragon in him.