It's not impossible. You can't evolve Eevee to those forms in Soul Silver. You have to trade it from Platinum already evolved.
Its impossible. And the other dude or dud-et that answered fails at eevee evolution's
You get leafeon by training an evee next to the moss rock in the forest and you get glaceon by trading from Platinum or pearl/diamond an you get glaceon in Platinum, pearl/diamond by training a evee next to the ice rock neer snowpoint city and its lake you also get leafeon on them by training in the forest next to the moss rock
You have to go to the stones that if at the forest or the snow place.
Get a evee in goldenrod city then go to get some kind of stone then you can evolve it