Quote Originally Posted by Mercen-X
I theorize that Vincent wasn't inside that coffin from the time of the experimentation. I believe that he had wandered off and possibly killed some people while in some state of delirium before he returned to his senses and to the Mansion he'd confine himself to. I think that he was perhaps injured in a fight and then sheltered by caretakers shortly before his return which would explain his outfit and his metal gauntlet. I think that while his mind was still plagued with the nightmares of Hojo's atrocities and the project he couldn't stop Lucrecia from participating in, he wandered into that room and climbed into the coffin as if sleepwalking.
I personally don't take it that far despite believing he wasn't placed in the coffin for the entire duration of those thirty years. I would assume he just got his outfit from rags and junk laying around the Mansion, myself.

As far as getting in the coffin goes, though, I would say Hojo did place him there, as Hojo's letter says he put Vincent to sleep in the basement.