I believe vocabulary is the most comprehensive and most difficult aspect of English for foreign learners to master thoroughly. They should concentrate first on learning the most frequently used and therefore most important English vocabulary for their practical real life needs.
Multiple sense English words and synonyms (words with a similar meaning) present special difficulty for foreign learners. Other difficulties with learning and using English vocabulary include fixed word collocations, phrasal verbs, idioms, proverbs and regional differences in vocabulary usage. There are differences in English usage in English-speaking countries in terms of spelling, pronunciation, vocabulary and grammar. I know some specialised books dealing with British and American English differences.
English usage can also be formal and informal. Formal English is the language of the mass media, education, business, economy, commerce, technology, science, etc. Informal English includes colloquial, slang and dialect usage. It is harder for foreign learners to master informal than formal English vocabulary.
Thematic general English dictionaries combined with English synonym dictionaries are a valuable tool for mastering English vocabulary logically, comprehensively and intensively for real life needs of learners. Extensive reading on a variety of topics is another effective method for expanding English vocabulary knowledge by learners of English.
And the question is?