Did a quick test. For those who have already beat the witch or just want to skip ahead with the save file, there is an wooden elevator box to the right which takes you to the three portals. All of them leads to the same map, just different locations. The upper left hand of the corner of the map leads to the milking machine with the normal witch, speak with her, and go on to the left side of her and kick her to the milking machine. The location for the milking machine for Vivi would be the upper right corner of the map (hard to be precise on the location), same procedure to kick her in the milking machine, choose the purple dialogue. Still can't figure out how to get the fat witch into the machine.
It seems like that you can't do that as of now. But she was one of the suggested characters that are in a poll for people to vote on who they want to see in the machine.