As others have already noted, vocabulary is the most important aspect with regards to learning a language to a good-enough extent; that is, with regards to making oneself capable of constructing sentences (as non-grammatical as they may be, or as unconventional as they may sound to a native or to a fluent individual, in spite of being deemed "technically correct") that can be understood by other people competent in a given language.

However, vocabulary acquisition shouldn't be constrained to the typical or more salient nouns and adjectives. It is absolutely crucial to acquaint oneself with discourse markers (adverbs, prepositional phrases, etcetera), conjunctions, auxiliary verbs, and other less markedly semantic and more functional elements. The main goal throughout the entire process should be becoming capable of producing intelligible sentences right off the bat about anything that crosses one's mind, without many hiccups. Of course, grammar, phonetics, and phonology must be taken into account as well, and a good grounding on their mechanisms is indispensable for anyone that aims to be proficient.

I'm writing a manual on language learning right now, and I've spent a very long time thinking and testing many different hypotheses and methods I've come up with through the years. It's quite an interesting topic, but I reckon many of the considerations that would make one proficient in learning languages can also be applied to any other learning process (learning to code, learning to play a musical instrument...), so it is probable that adopting a more general perspective would be beneficial.