The Pokemon HeartGold Version for the Nintendo DS was the basis for the romhack known as Pokemon HeartGold VX: Vanilla Expanded.
Fans of Pokemon HeartGold who want more out of the game but don’t want to play a hack that completely transforms it into a different game are the target audience for this mod.
HeartGold VX expands the playable roster by making many postgame-only Pokemon catchable during the main game. In addition to that, there are Pokemon that are unique to the SoulSilver version.
The player is not the only trainer with expanded options, as other NPC trainers also use new Pokemon to battle against them.
In HeartGold VX, there are additional ways to evolve Pokémon whose evolutions can only be obtained through trade or in Sinnoh.
There are many other small changes as well, which you can read about in the database provided.