So I want an Espeon in my team for X and I am not sure if when it is breed to perfection, if I should EV train it from an Eevee or an Espeon. Does it even matter when I do it because I can just use a reset bag and 're - EV train' it?
It does not matter when you EV train it. You could give it 252 SpA EVs at Lv. 1 or Lv. 100 and the outcome would still be the same. So, to answer your question, it does not matter whether you EV train it as an Eevee or an Espeon. However, my advice it to EV train it first thing so that you can steamroll stuff early-game.