>>45291434HOME lists the games each Pokemon is obtainable in, and it doesn't list anything for Victini, meaning there is currently no actual in-game event that triggers the encounter. It's possible it was scrapped, or is intended to be added in a future update and they accidentally left the encounter data in, but either way there is currently no hidden trigger we haven't thought of yet, it just straight up can't be encountered at all (unless of course HOME itself is wrong, but if people could figure out you had to make curry on an island to get Keldeo, I'm sure they would've figured out Victini if it were actually in the game).
