On the subject of vacuum-proofing concrete, found this exerpt from a lab test on a concrete-additive company's website (http://www.usefiberconcrete.com/Radon_Gas.html), extolling the ability of their product to block gas transmission through a pour sample:
"The total void volume as estimated from the helium porosity test is 9.6%, with a grain density of 2.45 g/cm3. The air permeability for the core plug specimen is 0.262 md, indicating that the macro and micropores are very poorly interconnected. Although liquid permeability was not assessed in this evaluation, concrete with a gas permeability of 0.262 md is likely to exhibit water or brine permeability of <0.05md."
Best as I can tell from the accompanying graph, the permeability of the treated sample (at least on the more-optimistic end of the scale) comes quite close to that of "impermeable rock" (e.g. granite).