User blog comment:Crimson and Darkness/Volume 25 discussion blog/@comment-74.138.206.123-20180606044402/@comment-217.136.96.176-20180608094534

Vali's Balance Breaker was far stronger than Issei's Balance Breaker, as their base power was world apart. Vali is an Ultimate-Class Devil class Hybrid even without Divine DIviding. However, the Boosted Gear's Balance Breaker can boost his power several hundred times in a short instant before Vali can divide it, enabling Issei to fight by consuming a lot of mana. But he cannot remain in that form as long as Issei.

Of course, now Issei's potential is limitless so who knows? Vali was also blessed by Ophis and has Devils in DxD can become pretty strong in terms of potential (Sirzechs, Ajuka, Balberith, Verrine). Super Devil an transcend to God-class level. Vali is the descendant of Lucifer and Lilith. So Devil potential combined with the infinite isn't inferior to Issei.