The defining vocabulary for a dictionary is a small set of words which are used to define all other words in that dictionary - very useful for English-English dictionaries designed for those learners of English who already know the basics of English and who are thus ready to continue learning English without translating into their native language.
Let us take a look at some properties of such a defining vocabulary:
You can have a look at the Longman Defining Vocabulary - a list of about 2000 words that can be used to talk about anything and to define all other words.
How to learn the most useful vocabulary in English - easily and quickly
First get an English-English dictionary for English learners. Then start reading any text in English and look up the words which you don't know. As you read the definitions for those words you will be constantly seeing the defining vocabulary used in practice. Apart from learning the words which you have looked up you will be first of all learning the defining vocabulary. By using an English-English dictionary regularly for learning any words however difficult you will be forced to absorb the most useful English words. It's as simple as that.
This is especially effective when you copy the definitions and example sentences from the dictionary by hand. If you do so you will be actually writing a lot of correct and useful sentences in English. You will be developing good habits and making sure that what you learn is really absorbed into your head. Those definitions will contain ready-made patterns that you can follow to express yourself. You will learn to express yourself correctly and clearly.
In this learning method the actual effort comes down to looking up words in an English-English dictionary and writing them down with their definitions and example sentences. This is really something that you can do. And the time spent using this method is really spent on learning English because you will be constantly reading and writing correct and useful English sentences.