To effectively work with the clay slab construction for this project (tall vase), roll 1/4" slab several hours in advance, place on a wooden board and cover with plastic to allow the clay to firm up a bit ("soft leather hard"). The hump molds shown in this tutorial are wheel thrown bisque. The bisque enables water absorption which will a) slab will not stick to it and b) will firm the clay for proper removal within minutes. To see how to make very large slab projects, visit here
To see a video of the tapping / pushing the round clay slab on the hump mold, see short video clip here.