I’ve done the ‘pumpkin as a vase’ before, but never thought of using a cabbage.  I think this is a great idea and really quite simple.  Cut a hole in the cabbage large enough to hold a vase or container in which to put your flowers.  I would suggest cutting the hole on the smaller side first, then shave away until your container will fit firmly.  It doesn’t have to be completely contained in the cabbage either. If your floral arrangement will drape slightly over the edge if will cover the container.  Remember to cut a slice off the bottom so that your cabbage will sit evenly, and place on a plate or platter.