If you think about the flow of electricity at the atomic level. The electrons flow by changing the positions from one atom to the other from their valance shells.
The battery just provides the electromotive force or the electric field for the movement of the electrons in the conductor. it is due to the interaction of the electron with this field.
The free electron theory in metals will give you a great idea about the topic. The negative electrode will not contain the electrons but the electro chemical reaction causes the electron to flow and complete the circuit.
The electron may come out of the conductor tip even if the battery is not there by the effect of photoelectric effect. But the work function for that material is what determines whether the electron will flow in the vacuum.
And to give a precise answer to your question the dielectric constant of vacuum is a lot.