The Vent Blueprint is not really necessary to complete your mission in 24 Carrot Island, but it will help you just in case you get lost inside the underground pipe in the factory.
How to get the Vent Blueprint
Go to the factory area and jump up to the top of the factory, at the roof you will find a blue rolled paper on the window. Just pick it up and you will see a guide map on the left low side of your game screen.