Things You'll Need
Instructions
Start up a game of "Pokemon FireRed" or "LeafGreen" on the first Nintendo Game Boy, and travel to Celadon City in the center of the Kanto region.
Go to the Celadon Mansion, the large building in the middle of town. Go inside and talk to the man there, and he will give you an Eevee. Assign it to your traveling party of Pokemon.
Leave the mansion and go to the Celadon Department Store. Go to the third floor and buy a Water Stone from the cashier. You can also find a Water Stone in the Seafoam Islands area.
Select the Water Stone from your bag, press "Use," and then select Eevee. It will automatically evolve into Vaporeon.
Start up "Pokemon Emerald" on the second Nintendo Game Boy.
Go to a Pokemon Center in both games, and go up the stairs to the Cable Club. Link the two Game Boys together using the Link Cable; doing so will allow you to take part in battles together, and to trade Pokemon.
Select Vaporeon in the "LeafGreen" or "FireRed" game and choose "Trade" to trade it across to "Pokemon Emerald."