When is it ideal to feed my pokemon vitamins?

During which part of my training should I feed my pokemon vitamins? When is it most beneficial?

I've been doing some serious EV training and have heard some people say that it's best to feed them as early as possible or as late as possible. Does that mean I should feed them as soon as I hatch from an egg or at a certain level?

When I've done it from egg hatching, I see barely any gain to my stats. I know it raises the base stat but shouldn't I still see some gain to my stats at that time?