What is the device called that can alternate between the pool skimmer and the vacuum?

I think you are referring to a Jandy valve. The valve diverts water from the main drain to the skimmers. When vacuuming, turn the valve so it is pointing to the skimmer inlet pipe.

OR

It could also be called a diverter valve located at the bottom of the skimmer depending on the age of the skimmer.

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