https://gamefaqs.gamespot.com/poll/2092-chaos-division-round-1-tifa-lockheart-vs-vyse
As if Vincent's performance wasn't enough, Tifa only doing a few percent worse on Vyse than Cloud had done the year before only furthered the idea that maybe they were actually benefitting from Cloud and Sephiroth not being in the main bracket. We had come up with the Vyse Fodder Line after SC2K4 because he had been right around 20% on Link in 2003 and 2004. Naturally, he fell off a cliff right after that.
There were a few theories for why it happened. Some chalked it up to higher vote totals in SC2K5, but Vyse's match against DK in SC2K3 actually got more votes than his match against Tifa. Some thought he'd just overperformed against Cloud because of anti-votes, but that's just kinda baked into his natural strength (although Tifa has always been the weird exception to FFVII's "rise to heaven" trends, which suggests she doesn't get anti-voted the way the rest of them do. I wonder why...!). Some people argued that fodder just can't be trusted to perform consistently because they're fodder, and maybe there's some validity to that.
The most likely explanation to me is that the demographic shift that happened in 2005 just tanked his strength. Niche RPG characters were stronger in the early days of the contest (Aya Brea almost beat DK once upon a time, and she's been terrible the couple of times she's shown up since). Even someone like Ramza went from putting up a solid showing against Kirby to losing to Laharl and Hogger. These things happen.
https://imgur.com/WqDcNNq
https://imgur.com/89Z5jrB