Here's why:
* Vaporeon is an evolution of Eevee. While you can catch Eevee in FireRed, Vaporeon is one of its possible evolutions, but it's not obtainable in that game.
* Eeveelutions are introduced in Generation II games (Gold, Silver, Crystal). FireRed is based on the first generation (Red, Blue, Yellow), so it doesn't have the mechanics for evolving Eevee into Vaporeon, Jolteon, or Flareon.
If you want to get Vaporeon, you'll need to play a Generation II or later game.