Add 1 to all the regional numbers in that Dex (e.g. Victini is 1, Snivy is 2, etc.), then edit the Pokédex scripts to subtract 1 from the regional number when displaying it in that Regional Dex.
EDIT: Here's the script changes you need to make.
First, in Settings, create an array like REGIONALDEXSHIFT=. The 4 is the fifth Regional Dex defined (the first is 0, second is 1, etc.), and you should change that to whichever number it needs to be. You can put multiple numbers into this array to affect multiple Regional Dexes.
Next, go into PokemonPokedex. In def pbGetDexList, add the parts in red: