Both the previous answers (though correct) are somewhat misleading. What $\epsilon_0$ is measuring is the strength of the electric force. The force between two point charges is stated by Coulombs law, which states
$F_e = \dfrac{1}{4\pi\epsilon_0} \dfrac{q_1q_2}{r^2}$, where q represents their charges and r is the distance between them. Electric forces exist everywhere in the universe, and $\epsilon_0$ is just a fundamental constant.
You seemed to have the notion that an interposed material like water decreases this force, somehow blocks the electric field. The actual affect is the opposite: the presence of a material between two charges increases their attraction. Why?
Pretend we have a positive and negative charge separated by a metal conductor. The charges will polarize the material, causing some of the electrons in the material to move closer to the positive charge, like this:
Though the net charge in the dielectric is zero, the charges on the electrodes will feel an attractive force in addition to the attraction that already exists between them, due to the material.
Anyway, materials have a property called permittivity, which quantifies how much they increase the force between two charges ($\epsilon$). I prefer to think in terms of relative permittivity, or $\kappa$, which is a unitless number that gives the ratio between electric forces in a vacuum vs. through a material. By definition, for a vacuum, $\kappa = 1$. Various materials augment the electric forces by various amounts, but in all cases, they have values of $\kappa$ greater or equal to one.
Footnote: even in insulators, where electrons don't move between atoms, this effect is still observed, due to electron orbits being slightly skewed to one side of individual atoms.
Best Answer
First, it is not "well known, a vacuum balloon using the materials we have at our disposal is not possible, because of the sheer force they have to resist from the air outside." In our patent application (Akhmeteli, Gavrilin, Layered Shell Vacuum Balloons, you can find it at USPTO site or at http://akhmeteli.org/wp-content/uploads/2011/08/vacuum_balloons_cip.pdf ), we show that sandwich structures made of existing materials can be both strong enough to withstand atmospheric pressure and light enough to float in air, according to our finite element analysis. As for your idea, external charge can gather on the surface of your structure due to discharge in air, say, during thunder, and ruin your design.