I agree with Captaincoconut.
To start with something basic, two of the results from yeast processing sugar are alcohol and the formation of CO2. If there is no fermentation occuring, there is no CO2. Keep that in mind.
If I understand things right, the small vortex that a stirplate creates is there to facilitate an exchange between the surface area of the wort and the air in the environment. It is accomplishing the same thing as shaking a carboy or spraying wort, but it is doing it to the 10th degree. An air lock is like a liquid check valve, so if you use one, by design, it will not let air into the environment. Without air constantly being introduced to the environment, a stirplate is pretty much useless. So stirplate = foil topped, if no stirplate, air lock or foil would both work (although some argue the pressure created by the airlock once CO2 builds has a negative impact on yeast health, so it may be best to omit them from starters all together). While yeast is on a stirplate (and is constantly being fed oxygen) fermentation is not occuring. It is all growth. If a starter is not placed on a stir plate (and given enough time) fermentation is occuring, and CO2 is being produced.
As the Captain mentioned, the vortex from a correctly used stirplate should not be able to create a vacuum, and will not result in suckback. However.....
....I think the poster who mentioned the vacuum argument may have been confused by an event that can occur during fermentation. If the wort (which I guess at this point is technically beer) is cooled after it has already reached an equilibrium of CO2 in solution, the cooling action will increase its ability to "absorb" CO2, and will start to pull CO2 from the environment. This results in the vacuum that can empty an airlock or suck-back through a blowoff.
Similarly, this is where you hear brewers mention warming up a carboy 2 weeks after fermentation has stopped and "magically restarting fermentation" based on seeing airlock activity. In reality, warming the beer creates the opposite of the vacuum effect, and CO2 is forced out of the solution, and back out the airlock.
So I suppose that if you made a starter a few days ahead of time without a stirplate (and complete fermentation had occured) and used an airlock on your starter, and then put the entire thing into your fridge, the starter wort will cool and could suck the liquid from the airlock......but that is making a lot of assumptions, and that person is not really handling their starters correctly.
Again, feel free to correct anything I said if it was off base.
Joe