The typical conception of the language learning process looks something like this:
In reality, however, the process should ideally look like this:
At the center of the language is a core set of patterns, constructions, and principles about how the language works and how it is used. It is impossible to become fluent without knowing this core in its entirety.
Outside this core level is a range of generic terms, constructions, and idiomatic expressions. Most of what we typically consider "vocabulary" sits in this sphere.
Finally, there is a level of specific or nuanced language that includes technical terms, jargon, and specific speech variants that are peculiar to a specific region or profession.
Being able to distill the core of a language is an important skill to have. If you can do that, then you'll be able to get inside the language quickly, then spread out into the other spheres.