As the controller of the acting minion, you have the first option to play effects when an action is blocked. If you play Malleable Visage once the block is determined to be successful, your vampire with vicissitude will enter combat with the blocking minion. Obedience cannot be played in response to Malleable Visage because the vampire using the Malleable Visage is not an acting minion and Obedience can only be used against an acting minion. You cannot play Malleable Visage after the reacting vampire has played Obedience as there will not be any combat for the Malleable Visage card to cancel.