One of the most important thing for Students of English as a Second Language is CONTEXTUALISING. It's acceptable that lack of vocabulary is another common problem, but the most important to understand a text is contextualising it, understand the meaning and explore the grammar on the text. Definitely it's the major problem for students around the world, in particular understanding the complexity of the grammar structure. I recommend, (my own experience) practising English through LYRIC SONG and as well as exploring the grammar structure in the text, you can do that using the lyrics, both you should analysing the contextualisation, of course as much as you practise, you'll get it as soon as possible.
As a Self-Taught and practising English a long time, now I can contextualising and understand clearer a text and a Lyric Song.
In conclusion it's impossible memorising 20 thousand words, but you can get it during a conversation, in the classroom, a self-studying, reading, etc, but the most important to overcome the barriers of the Language. Do not forget, practise makes perfect.
