Vitor was in a career slump (4-6 record) when he lost to Overeem. This was during the time when his sister was kidnapped and he later found out she was murdered. I don't think he was any stronger than Poatan, and he was outgrappled by many guys in his earlier days (WW sized Sakuraba, Tito, Couture multiple times even after Randy turned 40, etc.)
Vitor had two really strong periods in his career--the first 3 to 4 years then his later resurgance as TRT Belfort....until his TRT was taken away. He was a very inconsistent fighter outside of these periods. Also, his "BJJ black belt" credentials from Rickson (given when he was like 18) was and still is highly questioned. It's likely they gave him a belt so 1990s fighters would avoid the ground and be forced to stand, which was his strongest area.