First off, thank you for showing me a shortcoming with my choice of metaphor. You're right that "vacuum doesn't suck", that pressure flows from areas of greater pressure to areas of lower pressure. As a retired Navy engineer, I'm aware of what happens with pressure differentials.
That being said, as far as we have yet determined, the area *outside* the expanding universe is - *must be* - emptier than the area within the volume of the expanding universe. Moreover, we are not yet aware of what comprises the "fabric" of the spacetime we know, much less what comprises said fabric of the medium outside our expanding universe. But again, we're pretty doggone sure it's *much* emptier.
And the greater the pressure differential, the higher the velocity of the flow from the higher-pressure region to the lower-pressure region.
In fact, *that* is a possible flaw in my theory, for as the universe expands, the "consistency' (for lack of a better descriptor) will be lower the closer it is to the boundary between the expanding universe and whatever lay beyond it. If that consistency is lower, then the pressure differential should be lower, and so the acceleration of the flow from greater-to-lesser areas should be lower, thus obviating my whole claim in the face of the observed phenomenon that the farther away objects are, the greater the redshift.
I simply don't know. It's going to require a lot more thought.
And I sincerely appreciate the criticism. I really do.