Why Is Vision In Avengers So Weak?

Firstly, Vision’s whole body wasn’t just Vibranium. In the movie ‘Avengers: Age of Ultron’ Ultron made it serve as his ‘Organic Body’ with artificial muscles developed by the geneticist Dr. Helen Cho. Therefore, only the outermost layer of Vision’s body was of Vibranium.
Secondly, Thanos is one of the most powerful beings in the Marvel Universe. He carried the Infinity Gauntlet as if it were a hand glove. He also had other infinity stones with him that added to his power.