User blog comment:B214/Question & Answers 5/@comment-82.199.207.24-20160423084304/@comment-82.199.207.24-20160424103644

It wasn't Vali who enhanced the power of his attack it was Rizevim who imbued his fist with demonic-power to hit Vali in the abdomem and Vali coughed up blood after taking the hit.

And regarding the illustration it is clearly showing that Issei is concentrating his demonic-power in his right fist to enhance his strength and as a result he smashes Raynare through the wall.