This is osmosis. Vinegar isn't pure acetic acid...it's a 5% solution in water. The concentration of water is higher in the vinegar than it is in the egg...so via osmosis, the water moves from the area of high concentration (outside the egg) to the area of low concentration (inside the egg) causing it to grow. Of course it helps that the acid (vinegar - acetic acid) will dissolve the egg shell (calcium carbonate) as well.
