Re: Why liquids don't move into a vacuum, but gases do.

Well, Liquids and Gases do move into vacuum. Liquids and Gases are termed fluids and they are intended to just behave the same except for matter of how close they are together.

Vacuum, literally means a volume that has no pressure (O N/m^2 or 0 lbs/ft^2) . What that means is that, it is an area with absolutely no mass but it has dimensions. When something that has mass comes close to vacuum, it tries to get to the lowest state possible (lowest density) (refer advanced laws of thermodynamics.)

So now you know that both liquids and gases do indeed move into vacuum.

Now answering your question: I am not sure what situation you are referring to but generally since liquids are heavier than gases, the gases move into occupy the vacuum quicker than liquids, so the apparent observation of gases flowing into vacuum, when liquids don't.

If you are not convinced, you'll need to try it out:

Please note that you'll need to use a wet or wet/dry vacuum cleaner.
Also have a parent/guardian watch you when doing this experiment.
Do not use anything other than water (no oil, alcohol etc.)

Get a vacuum cleaner, hold it 6 inches on top of a bowl filled with water, and switch on the vacuum cleaner. You will notice that the water moves, and may be small droplets are sucked in, but definitely not much.

Next you dip vacuum cleaner into the bowl of water and switch it on. You'll see the water level lowering. This is because the water was attracted to the "vacuum" that was created.

Hope you have understood the concept. If you haven't please post your exact situation and I'll try explain it as best as I can.