What is the difference between verduras and vegetales? In what situations can one be used as a translation for "vegetables" and the other cannot?
Vegetal (here the new definition according to RAE) is either:
Verdura, on the other hand, refers to edible plants, specially those with green leaves.
In Spanish (in Spain, at least) we make a distinction between hortaliza (any vegetable, in the 2nd meaning of vegetable as a noun in the previous link) and verdura, which are a subset of hortaliza.
So lettuces, spinachs, cabbages, caulifloweres, chards, etc. are verduras, but potatoes, carrots, tomatoes, etc. are just hortalizas. However, when someone says "es bueno comer verdura", we all understand that carrots, tomatoes, etc. are included.
A "vegetal" is anything relative to plants, and "verdura" is a vegetable. So a "verdura" is a "vegetable", but a "vegetable" is not necessary a "verdura".