It's quite an excellent idea to have Hojo steal Gast's video equipment. For one thing one makes immediate comparisons with the innocence of Gast's project, videotaping Ifalna and his child and the perverted nature of Hojo's experiments on his own child.
Sephiroth's excessive solemnity before the camera and his suspicion of it are excellent details. The fact that Sephiroth only speaks when he is told to is also telling and chilling.
That Sephiroth's first word is "mother" and that he asks where she has gone is especially poignant and significant.
I have never seen Sephiroth's character explored in such a gripping way before. It's remarkable because scientific tapes have the potential to be rather boring, but by only selecting the most interesting pieces of them through Vincent's point of view, you make a wonderful story.
Vincent is of course an excellent choice for a perscpective on Sephiroth because he has more reason than many to be sympathetic to him, also being the victim of Hojo's experimentation.
You write beautifully. Vincent's musings about time are so wonderfully written. The end of this story is poetic. The last lines really resonate.