Vincent, again.... [spoilers, maybe]

[Squall of SeeD]

Who decided he must sleep there? Was it himself?

Hojo's letter in the Shinra Mansion has him stating that he put Vincent to sleep there, but Vincent very obviously felt that he deserved it as a punishment:

(From Hojo's letter.)

"I must get rid of all those that stand in the way of my
research. Even that one from the Turks."
"I scientifically altered him, and put him to sleep in the
basement. If you want to find him, then search the area."

(From Vincent's dialogue in the Shinra Mansion's basement.)

Vincent
"There was no way to cancel the experiment."
"I couldn't stop her."
"That was my sin."
"I let the one I loved, the one I respected most, face the worst."

Tifa
"So the punishment was sleep? That's weird."

How long had he been sleeping there? Since Hojo experimented on him?

We're not told, sadly, though I doubt that was the case. When Vincent awoke, he was still wearing his Turk uniform and Hojo was nowhere in sight, nor did there seem to be an indication that Vincent was confined:

Was he sleeping during the burning of Nibelheim? The mansion didn't burn, did it?

Again, we're not told when he was put to sleep or for how long, so it's impossible to determine, really. No, by the way, the Mansion didn't burn.

If he was there at that time, did Sephiroth know about him being there?

Certainly not. Had he, he would have no doubt questioned him about the past and his possible involvement in his origins. But, again, it may be that Vincent wasn't in the basement yet.


In all honesty, considering that the letter Hojo left in the Shinra Mansion isn't there in the Kalm Flashback, that Vincent wasn't wearing his red outfit when he awoke from Hojo's experimentation and didn't appear to be confined, nor did Hojo seem to be around, and that Hojo's letter doesn't say exactly when he put Vincent into the basement, I think that Vincent wasn't placed there until after the razing of Nibelheim by Sephiroth.

There's no strong evidence for it aside from the massive holes in narrative concerning Vincent's past, but it seems to be just as likely. Perhaps Vincent attempted to stop Hojo and the Shin-Ra soldiers from rounding up the survivors of the massacre and was incapacitated, then placed into the basement where Vincent accepted it as his punishment.