Hey guys,
I'm having the exact same problem
I have the luxury of 2 PC's, one with vista and one with xp. So I've been switching save files whenever the game crashes in vista.
I've found, that Seifer is not the trigger for the crashes. In some parts the game crashes with him, in others he does just fine. The game also crashes in other parts, unrelated to Seifer.
In 99% the game crashes during cutscenes were the player has no control over the game. Only when entering Sacred's cave did the game crash without a cutscene. There seems to be no specific animation that triggers the crashes, no different animation. A note of interest: At one try the game did go past the first Seifer crash, only to crash after Cid did his speech.
The game crashes in vista with or without the patch 1.2, with or without the launcher, with or without moving the exe file, with any configuration of the configure file and with all compatibility modes. Compatibility 95'or 98' is not possible, game crashes immediately. In Xp the game did not crash once!
Is it possible the crashes may be sound related? Have not seen any research there. The installer of Yamaha crashes when trying to install in all compatibility modes.
Hope this info helps to find a solution. Love what you guys did with the graphics and love this game.