have you looked at the implosion engine designed by Viktor Schauberger? you might find it interesting...
to my mind, vacuum energy doesn't refer to pressure differences as you describe, it's more about strange quantum effects that can occur even within a pure vacuum... photons of EM radiation can transform into shortlived electron/positron pairs that quickly destroy themselves releasing a new photon.. i think perhaps this is what people refer to when they speak about vacuum energy...if this process could be manipulated to produce an extractable electron current, energy could be drawn from the vacuum, the "virtual particles" constantly bubbling into existence within the dirac sea...