I was looking at the Collins Cobuild Compact English Learner's Dictionary for the word 'patato' and it had this definition:

Potatoes are vegetables with brown or red skins and white insides.

Why 'vegetables' and not just 'vegetable' when we are talking just about one kind of vegetables, potatoes?
1 2
Comments  (Page 2) 
Nona The BritNo, broccoli is not a plural - 'broccoli are vegetables' is incorrect.
I see your point.

Sounds like a bad translation from Italian to English to me.

Yoong LiatIf I'm not mistaken, Believer's question has not been answered.

He wants to know why, according to the Collin's Dictionary, 'Potatoes are vegetables with brown or red skins and white insides' and not 'Potatoes are a vegetable with brown or red skins and white insides'

Potatoes are vegetables.
is correct, because we're talking about all (or many) instances of potatoes; you could equally say:
All potatoes are vegetables.
We're not talking here about something generic, representing the species.

If you want to talk about the species, you must talk about "the potato" and then you must use the singular for the vegetable:
The potato is a vegetable.
Students: We have free audio pronunciation exercises.

Can we say "Potatoes are a type of vegetable ..." Wouldn't it be better?
This is an interesting discussion! Thanks to the smart question.