Disclaimer: saying that vocabulary is a primary metric for life "success" is in no way saying it determines it. There are no determining factors for success. And success is circumstantial, defined individually. However, success can be measured for any definition of it, and there are underlying causes in any example of success.
After a century of extensive research on aptitudes, the Johnson O'Connor Research Foundation has become known to be the world's leading aptitudes research institute. Their aptitude testing has been invaluable to me in my own journey of learning how to apply mine. A significant part of the founder's research observed people with noteworthy levels of success in their respective fields from a wide variety of backgrounds and tried correlating their achievement with as many factors as he could - gender, age, education attainment, and more. O'Connor said that command over vocabulary "accompanies outstanding success in [the United States] more often than any other single characteristic which… the Johnson O’Connor Research Foundation has been able to isolate and measure."
The most sited study by Johnson O'Connor conducted was extensively completed with the employees of 39 large manufacturing companies. Below are the results of a vocabulary test that have been averaged and grouped by the levels of their common corporate hierarchy. The relationship between employee's vocabulary and their position in the company demonstrates that better ability to think and commmunicate is closely tied to professional success.
There were no vocabulary tests administered in the hiring processes for the Foremen, Superintendents, Managers, Presidents and VPs. This trend was incidental. Their verbal command improved their ability to think, administer, and lead. The findings on this may be surprising. That's because we do not emphasize the importance of rhetoric enough.
People typically over-emphasize the relationship inheritance plays with "success" (however they define it). In reality, the good majority of children of wealthy parents do not grow up to become wealthy as well. This phenomenon has been discovered in so many studies in so many ways. Search it - or read The Millionaire Next Door. Money has incredible entropy. The difference between inheritance and increasing one's vocabulary, however, is significant, because one's ability to think is where personal success originates.
Few of us live with any significant lack of great reading material.
The reasons why command of vocabulary accompanies success so closely should not come as a surprise. Words are the very building blocks of thought. Expanding your vocabulary enhances your thinking capacity, which benefits all areas of life. The same way mastery of a wider variety of tools opens possibilities exponentially for an engineer, mastery over the building blocks of thought enable us to increase our level on consciousness, creativity, and leadership capabilities. We're able to build more out of ourselves as well as the conditions of our lives.
See the illustration here about how the results of increased vocabulary compounds.
We know the compounding affects of increased vocabulary to be real because it is the same pattern we see in early childhood development. As babies and toddlers absorb words from their environment and utilize them, they exhibit increased control over their lives. The difference between babies and (most of) the rest of us is that we lose our eagerness to learn. For those that maintain their commitment to improved rhetoric, they won't use commonly misused words. Their words carry added levels of thought. They speak signals in a world of noise. When they speak, people listen.
The difference between the almost right word and the right word is really a large matter -- Mark Twain
'tis the difference between the lightning-bug and the lightning.
Neural networks are the wiring of cognition. They evolve around the thoughts that we reinforce - our interests.
The effectiveness of any of the following points will only go so far as your interest takes it. Even a strict vocabulary memorization regimen will yield few results if your heart isn't in it. You won't read voraciously without interest. You won't look up words without a curiosity for words. New words will be forgotten because they weren't relevant to what you think about.
Learning new words is most effectively done within the context of topics you care about, because you'll think about them and use the new words with less realization that you're practicing them. When working within your interests you will reinforce the neural pathways surrounding the new vocabulary and more easily retain them in memory.
While you read, you're increasing your mastery of the words you already know as well as learning new ones. The difference between reading and watching videos is that watching can easily devolve into passive observance - and unwhitting mental conditioning. The less a person reads the more they are mindlessly accepting ideas presented to them at face value. Reading is more active. You can move at your own pace, make sense of the words, and think through the ideas presented more deliberately.
Disclaimer: The effectiveness of what you read will only go so far as the quality of its writing. Writing that is produced to spread gossip or sell products will do little to nothing for building one's thinking capacity.
Referencing new words in a dictionary during the course of your reading is the best way to learn new vocabulary. This is because whatever topic you were reading about either created a new neural pathway or strengthened an existing one that you can attach that new word to.
To gain the benefits of habitually looking up words, you must cultivate a curiosity for words. Learn word roots, even a little latin, and the historical context surrounding phrases and sayings.
As you do this, the manners of speech people use in even the most ordinary contexts will carry weight you previously would not have appreciated. Carrying an interest in words with you will make you a more deliberate listener, because you'll pay closer to attention to the words people use. You will experience life more richly.
After reading, the best way to build your rhetoric is to write. Reading and looking up words are inputs. Writing is practicing your output. Nothing improves your rhetoric like writing does, because it sorts out your head. The most articulate among us have a disciplined habit of writing. Writing is a natural context for utilizing new words. It's much easier and more judge-free context than doing so in conversation with someone else.
This is the most direct way of building your vocabulary. This approach does not, however, give new words the personal relevance that running into them within the flow of exploring an interest does. It is true, though, that after learning new words the universe seems to have a way of presenting them again to you. This is just because you'd notice it in a way you otherwise would not. My favorite methods of memorizing new words are using the mobile app for Vocabulary.com and, on occasion, spending some time referencing the thesaurus. Learning new words and contrasting them with words you already know sets you up for success the next time you just need that perfect word for the situation.