Context would tell if the action is in progress at the time you are speaking or if it is a habit of the eater's.
Alternatively, the French use a phrase to replace the continuous verbal form:
"tu es en train de manger des légumes": "être en train de + infinitive" means "to be in the process of doing something".
"du" is a partitive article used to mean "an unknown quantity of" in front of an uncountable noun in masculine and starting with a consonant sound: du riz, du vin
"des" is an indefinite article used to mean "an unknown number of" or "more than one" in front of a countable noun: "des" is the plural of "un" or "une": un légume, des légumes; une tomate, des tomates.
In the strict, technical sense, it does! [see end]. In the common sense it includes more than that. Still, the translation to legumes should be accepted since IT IS also true! (just like for j'adore both "I love" and "I adore" are accepted ) wikipedia: La gousse ou légume est un fruit sec qui s'ouvre par deux fentes, isolant deux valves, caractéristique des plantes de la famille des Fabacées, souvent appelées aussi des légumineuses pour cette raison. -- same definition for legumes in english!
I was marked wrong for typing "you are eating a vegetable" instead of "you are eating veggies". My question is how can you know if a sentence is singular or plural from just listening? I know some situations can be obvious, but in this scenario, isn't "you are eating a vegetable" correct also?