I am relatively new to ev training and I have given my eevee 252 ev's in special attack and speed, would leveling her up by battling upset the balance of the ev's, or would she not gain any ev's?
She has 4 evs left if you gave 252 SpA 252 Spe so that means if the pokemon battling gives of 4 evs other than SpA as an EV and speed than you would gain those after you defeat the pokemon and wouldn't be able to gain more. the mex limit is 510 but 2 evs don't matter.
In other words, (What the pika said) you still can give 4 EVs to you're Evee. Then It won't matter what you battle, because you're maxed out in Effort Values.