no you just have to see them for them to count in the pokedex... but if you want to catch them, you can take the eevee you eventually trade for, and put it in a daycare with a ditto... they ditto and eevee get it on and you get an eevee egg... if you make it happy during night you get an umbreon, happy at day, an espeon, and then you use stones to evolve them into the other three evolutions