Hmm...
<You can find it in gears>
Save file consists of a 0x09 byte header and 15 save-game slots. Each save-game is 0x10F4 bytes large, so first save-game is at 0x09, second at 0x10FD etc...
Vincent/Sephiroth record and first byte which determines who's record is it, can be found at 0x03F0 counting from the beginning of the save-game. So...
These addresses are: 0x03F9, 0x14ED, 0x25E1 etc...
If that byte is set to 0x0A then it's Sephiroth's, and if it's 0x07, then it's Vincent's record.
So basically (assuming that it's a first save-game in save-file):
You have to write a byte 0x07 at 0x03F9 (1017 decimal).
As for the Hex Editor... I use "Magic Trainer Creator" since that was the very first program which I used to change things (memory/raw files), but it doesn't have friendly interface...
dziugo