Vegetable Vaughn is a Zorg Commander who has green skin. Why he's fighting against Mental is unknown. Based on his quote in the character select screen, Vegetable Vaughn is an ardent vegetarian, which explains his name.
Vegetable Vaughn, using a recolored version of the Serious Sam HDZorg Commander model, can be found in the Serious Sam 3: BFE game files. It seems that Vegetable Vaughn was supposed to be playable at one point, but was cut for some reason. However, with some file editing, he is fully playable.