Can energy be borrowed from vacuum? Even if virtual particles do not really exist does energy in vacuum exist in reality? Is the energy that can be borrowed from vacuum to return it inmediatly real?
 A: The vacuum energy is generally referred to as zero point energy and yes it does exist. Well, actually we have some evidence that it does exist and some evidence that it doesn't. It's likely we don't currently understand this issue thoroughly.
The evidence that vacuum energy exists is the Casimir effect. In the absence of any constraints the Standard Model vacuum has some state that we'll call $\lvert\Omega\rangle$. If we place two metal plates very close together then this perturbs the vacuum state and in between the plates the vacuum state is slightly different - lets call this state $\lvert\Omega'\rangle$.
The zero point energies of $\lvert\Omega\rangle$ and $\lvert\Omega'\rangle$ are slightly different, and the result is that there is an attractive force between the plates due solely to the difference in zero point energy. This force is measurable and indeed has been measured, so we know the Casimir effect is real and therefore that zero point energy is real.
On a side note: this is not a result of vacuum fluctuations since as pointed out in your previous question the vacuum doesn't fluctute. The effect is due to a difference in the vacuum states not some hypothetical fluctuations.
The problem is that this zero point energy should have a gravitational field, and in fact that field should be so strong that the universe as we see it today could not exist. This constitutes evidence that zero point energy does not exist.
Which leaves us in the uncomfortable position of not being sure what is going on. I think the general view is that zero point energy really does exist, but for some reason we don't currently understand it does not gravitate.
