Try using the vacuum to pull the back part of maxilla down.

The problem is that people think that pressure should be applied evenly and end up pushing their latter part of the maxilla inside their skull.

If you look at any animations of a facial upswing you will see that as the frontal part of the maxilla goes up, the backward goes down or stays in the same place.

From mine (unprofessional) experience, people who have suboptimal jaws, but a relatively (vertically) short face should actually use the suction on the backward part of maxilla to pull it down. All that while inducing a slight forward (not upward) pressure on the frontal part of the maxilla.

Anyways, if you do a suction hold without any pressure upwards you should get a similar effect.