Lecithin:

“ Lecithin is a fat that is essential in the cells of the body. It can be found in many foods, including soybeans and egg yolks. Lecithin is taken as a medicine and is also used in the manufacturing of medicines. Lecithin is used for treating memory disorders such as dementia and Alzheimer's disease.”

Uses, Side Effects, Interactions, Dosage, and Warning

As is evident from the above description, Lecithin is a fat, and can be sourced from a variety of plant and animal products. The most common sources are soy bean and egg yolks. It is a common food additive and is used as an emulsifier. It’s name is derived from the Greek lekithos, meaning egg yolk. The substance was first isolated in 1845 by Theodore Gobley and was sourced from an egg yolk.

Lecithin - Wikipedia

To determine Lecithin’s suitability for a vegetarian or vegan diet the source from which the Lecithin is derived is crucial.

According to Wikipedia:

“Commercial lecithin, as used by food manufacturers, is a mixture of phospholipids in oil . The lecithin can be obtained by water degumming the extracted oil of seeds.”

and:

“… a gradual shift to other sources of lecithin (e.g., sunflower oil ) is taking place. The main phospholipids in lecithin from soya and sunflower…”

As others here have wisely provided, though it is possible, and even likely, that consumable lecithin is plant sourced, it is still a highly processed ingredient and it consumption should be considered carefully when planning a healthy nutrition regime.