The FDA as of 22 June, 2009 was granted the authority to regulate tobacco products. The part of the FDA that regulates tobacco products is known as the center for tobacco products. This department was officially established to implement the tobacco control act. The purpose of regulating tobacco products is to use the best available science to put an action in place to effectively help the public reduce the toll of illness and death caused by tobacco. For a product to be deemed “tobacco product” it has to be derived from tobacco that is intended for human consumption.
In 2016, the FDA finalized a rule extending regulatory authority to cover all tobacco products including electronic nicotine delivery systems.
The e-cigarette should not be considered a medical device since currently it has not been used to cure any disease. Per the FDA a device should be able to be used to diagnose a disease or other conditions, or used in a cure, mitigation, treatment or prevention of a disease. It also defines it as an accessory that is intended to affect the structure or any function of the body of man as long as it is not achieved through chemical action within or on the body of man. For a device to undergo clinical trial it must have the capabilities of curing a disease or helping one maintain a comfortable life due to some illness. The reason an e-cigarette is not classified as a class III device is because it is not considered to be a high risk device and therefore not subject to highest level of regulatory control.

https://www.fda.gov/aboutfda/transparency/basics/ucm194423.htm
https://www.fda.gov/AboutFDA/Transparency/Basics/ucm194439.htm
https://www.fda.gov/TobaccoProducts/Labeling/ProductsIngredientsComponents/ucm456610.htm#regulation