Version Exclusives are certain type of Pokemon that can only be found in one version of any two Pokemon Games that are released as a set. In regards Pokemon: Let's Go, Eeevee, these are Pokemon that can be found within the game that cannot be found in its counterpart, Pokemon: Let's Go, Pikachu. However, in that same sense, there are certain Pokemon that can be found in Pokemon: Let's Go, Pikachu that cannot be found in Pokemon: Let's Go, Eeevee. In order to obtain the Version Exclusives from the game opposite of the one you own and thus complete your Pokedex, you will need to either trade with a person who has said other game or transfer the Pokemon you can't find in your game from Pokemon GO.
Pokemon: Let's Go, Eeevee Version Exclusives
Arbok
Vulpix
Nintales
Meowth
Bellsprout
Weepinbell
Victreebel
Koffing
Weezing
Pinsir
Eevee (Only Starter Partner Pokemon, Not Wild Eevee In General)
Pokemon: Let's Go, Pikachu Version Exclusives
Sandshrew
Sandslash
Oddish
Gloom
Vileplume
Mankey
Primeape
Growlithe
Grimer
Muk
Scyther
Pikachu (Only Starter Partner Pokemon, Not Wild Pikachu In General)