Question: is issei stronger than vali?
Answer: While Vali uses his lifeforce to maintain DXD L. Summoned Draig is said to be equal to or stronger than Issei in his DXD G form. Vali definitely has a better technique and speed but Issei has a better power output even in his pseudo DXD form. So there some to no chance Vali can beat Issei for now.