If the vacuum is trying to pull the piston back into the cylinder, it must have a negative pressure, since a positive pressure would tend to push the piston out. Then the vacuum energy density stays the same, but the matter energy density goes down by 3 parts per million. Any small deviation gets magnified, and the model is fundamentally flawed. In addition to this flaw of instability, the static model's premise of a static Universe was shown by Hubble to be incorrect. This led Einstein to refer to the cosmological constant as his greatest blunder, and to drop it from his equations.
M are very hard to solve exactly.
With a large amount of mathematical work it is possible to prove that the ground state of this system has an energy that is less than infinity. But there is no obvious reason why the energy of this ground state should be zero. So there must be a suppression mechanism at work now that reduces the vacuum energy density by at least 120 orders of magnitude. We don't know what this mechanism is, but it seems reasonable that suppression by 122 orders of magnitude, which would make the effect of the vacuum energy density on the Universe negligible, is just as probable as suppression by 120 orders of magnitude. So in the inflationary scenario the vacuum energy density was once large, and then was suppressed by a large factor.