In the National Pokedex, Victini comes before the Unovan starters, not the Hoennian starters. It comes after every Pokemon introduced prior to Generation 5.
In the Unovan regional Pokedex, it is unique in the sense that it is the only Pokemon as of Generation 8 to have a Pokedex number of #000. It comes before the Unovan starters because the developers claimed it was special enough to deserve a unique placement. It being #000 means you won't see an empty Pokedex entry when you get your starter, helping keep the mystique around it.
Of course, that doesn't mean they couldn't have just put it at the end of the Pokedex like all the other "special" Pokemon.