I actually just did this with a white labs vial. Started the starter last Wednesday. I left a tiny amount in the vial....maybe the amount you could get from one Belgian bottle. I poured some of wort I had just made for the starter into the vial, closed tightly and shook. Then I loosened the cap until I heard air escape and left it at that tightness. I repeated the tightening, shaking and untightening whenever I walked past. And I also added more wort whenever I stepped up the starter. There was about a half inch to inch of yeast in the bottom of the vial after a few days.
I assume there wouldn't have been much difference than just filling it up with the starter, but I was interested in seeing how little yeast I could get away with as I'll probably try it with a Belgian bottle at some point.