Do not think of vocabulary as vocabulary. Think of it as knowledge.
A real example from class
One day my class read a piece about urban farming, about why people in Paris are doing it. The text carried some lovely language: hydroponics growing system, vertical farming, vertical tubes, underground farms, rooftop farms.
Instead of handing out a word list to memorise, we read the piece and then talked about what it said. Is this a good idea? Does Saigon already have anything like it? What about your city?
It was purely an exchange about the subject. But talking that way, those words went in without anyone noticing. And the next time the topic of agriculture came up, they arrived naturally, with none of that strained feeling of retrieving a word you crammed for this topic.
Why word lists fail
Most people take a "band 8 vocabulary for farming" list, memorise it, and try to deploy it.
That is extremely hard to retain, because you have no emotional connection to those words. Often you have no logical connection either, because you do not really understand what they describe. Recite like a parrot and you will certainly forget.
Vocabulary needs something to attach to. That something is an idea you found interesting, a story you have an opinion about. Learn the knowledge and the words come with it. Learn words with no knowledge behind them and there is nothing holding them in place.