Yes, "vous mangez" is plural. It is the French second person plural form of the verb "manger," which means "you eat" or "you are eating" when speaking to more than one person.
mangez-vous aujourd'hui ?
"You eat breakfast."
je vais manger
'que vous mangez des changements'
Qu es-ce que vous mangez ?
vous mangez (formal) tu manges (informal)
Translation: (informal - tu) Mange-le / (formal - vous) Mangez-le
(Formal) Vous mangez (Informal) Tu manges ("Tu manges un croissant" = "You eat a croissant.")
Je mange (I eat).
mangez is the 2nd person formal and/or plural (vous) conjugation of manger (to eat). It can be a command (imperative), such as "mangez votre dejeuner" ("eat your lunch"), or the present indicative (a question or statement of fact in the present tense), such as, "Vous mangez votre dejeuner rapidement" (You eat your lunch quickly). mange is both the first person singular (je) and third person singular (il/elle/on). It is indicative only; examples for use are: "Je mange souvent du pain" (I often eat bread) or "Il mange du soupe quand il est malade." (He eats soup when he's ill)
"Avez-vous faim ?" or "as-tu faim?" if you already know the person.