IME VTs are a laminar flow pump that can pulse, and if used in pair(s) they can create a back-n-forth laminar flow. WBs are strictly back-n-forth... I've seen one on with NO other pumps on in the tank, and it literally picked up detrius off the bottom and moved it back-n-forth slightly (hovering motion if you will). WBs need additional laminar flow, VTs do not.