Dunno if I'm right, but part of me figures she was replaced at the very beginning. Y'know, when she woke up not remembering the night before. That's my opinion though.
Though that wouldn't explain the whole vision from the first though. Still seems like the most likely time.
Issue 8 explains everything. Spoilers below.
Issue 8 tells us the real Buffy got replaced at the beginning of season 9 during her drunken party. Buffybot has thought she was the real Buffy the whole time. We also learn the real Buffy is a housewife (we don't see a family, IIRC), and has no idea who she is, and gets kidnapped by a slayer.