When learning languages, its sometimes argued that you should on only the top 1000 or so words. That's because most common spoken language really only uses these words. If you focus all your time on just these top words, you'll get to conversational level which will be hugely motivating for getting to the upper levels of learning a language.
I'm not sure this actually words in practice though. The pareto principle is principally applied to cause-and-effect scenarios: how can I do 80% of the word with only 20% of the effort? Vocabulary is a big more nuanced than that.
Why doesn't it work?
For one, the top 1000 or so words is not always a list you can find. I'm currently trying to learn Lithuanian -- if I go duckduckgoing for "top 1000 words lithuanian" I'll get links like this.
This list is not very good.
Take the word bũti: this is the very used "to be." It needs to be conjugated for future tense, present tense, past tense when the action complete (past simple), when the action is not complete or redundant (past complex), and a host of other use cases. It's total conjugation table is daunting for a native english speaker such as myself:
The top 1000 list radically takes only a few forms of it: buvo (he/she/they were), yra (he/she/they are), and bũti (to be, infinitive).
Is it redundant to include all the conjugated forms? Perhaps its lacking to only include one form?
Similar to this, and piggy-backing off "Latin by the Dowling Method", not knowing grammar will mean you can't deconstruct the sentences. You're trying to brute force memorize a bunch of random sounds and letter combinations as opposed to learning the language.
That's because languages are a way of thinking. You need to get to the point where you can reflexively pick out the function of a particular noun or verb in a given sentence.
That is easy for Spanish, which has a strong word order of subject-verb-predicate for most sentences with few exceptions. Balto-Slavic languages have relatively free word order where the ending of a word dictates its place. If you manage to memorize all those words, you'll be struggling to figure out sentence meanings because of the relatively free word order.
And that's if you manage to memorize all of them. Not knowing the context or place of words makes remembering them much harder. Some places will argue that you can use some mnemonics but that takes you out of thinking in the language and instead remembering them using your native language. This is no good.
What does it work?
Focus on trying to talk and write with a complete sense of grammar and a small vocabulary. As you struggle to explain exactly what you want, you'll learn how to mime, gesture, and say it with a more simple vocab. This will help reinforce grammar and get you thinking in the foreign language. After a while, you'll come up with your own list of vocabulary to use.
And to get that high level, you may need to brute force the grammar. I've been doing this lately with my Lithuanian and, everyday, rewrite most of the grammatical charts for noun declinations and verb conjugations. Lithuanian is a particularly complicated language and this will likely be much easier for a language like Spanish whose grammar is closer to English (unless you're learning Tibetan or Georgian).