We've been vermicomposting with worms for a while now and have beautiful worm castings made from our produce scraps, used coffee, and paper towels.
Because the worm castings are full of worms, and we don't want to loose any of these compost workers we bulk harvest the finished vermicompost every couple months separating the worms from the castings. To do this we simply make a pile twice the size of the amount of castings we want and shine a light on half of it. Within 8-12 hours almost all of the worms move from the side with the light to the side without the light. Then all we do is just take the lit side of the pile for our plants and soil, while returning the other half, with all the worms, to the composting tubs.