Phrase Differences
Defining Vocabulary: This means vocabulary which defines. In context, this refers to the vocabulary used to define dictionary entries. For example, it includes all words in the phrase "Neither causing nor exhibiting symptoms of disease." for this definition:
asymptomatic: adj. Neither causing nor exhibiting symptoms of disease.
Vocabulary Definitions: This means definitions for vocabulary. In context, this refers to specific definitions for dictionary entries. For example, it includes the definition "Neither causing nor exhibiting symptoms of disease," but does not include lone words from that definition.
Rephrasing the Sentence
The defining vocabulary of dictionaries are restricted to a range of between 2000 and 3000 of the most frequent used word families.
This is incredibly awkward and misinterprets the meaning of the original sentence.
This reworded sentence states that all dictionaries have a defining vocabulary which must consist of only between 2000 and 3000 "word families", a phrase which is not defined. This does not account for other possibilities allowed in the original sentence, such as native speakers' dictionaries, which have defining vocabularies much higher than 3000 words, and the fact that learners' dictionaries don't need to have defining vocabularies within the specific range of 2000 to 3000 words, as I'm quite certain that at least one learners' dictionary's defining vocabulary is outside that range. If I were to restructure the sentence, I would write something like this:
Practially all English learners' dictionaries have a defining vocabulary that ranges in size between 2000 and 3000 words.
And "defining" is a verb.