If someone tries to learn language, he must master at least some essential vocabularies for daily use for speaking activity. However, the purpose of learning foreign language are many. If someone only wants to use the language for daily conversation, then he doesn't need to master all vocabularies but only the selected ones like conversations held in the market, post office, school, and many more.
When someone wants to learn technical terms or scientific purpose, then he must increase the capacity of his vocabulary mastery. For information, there are many levels in achieving language like functional and informational level. In functional level, he is only able to speak in a daily situation. In informational level, he is able to read the academic books.
There is someone who is working abroad and able to speak fluently in daily conversation but not able to read the academic books which contains complicated vocabularies. However, the scholar, who is able to read some complex vocabularies from the text books, it doesn't mean that he is also good in speaking.
However, in my opinion, reading is still the best way to increase vocabularies since vocabulary mastery is one of the most important key to learn foreign language. According to some expert, a foreign learner is almost impossible to speak as well as a native one because the learning process begins from a very young age for native.
By reading book, especially novel, the learner may know some dialog, expression, utterance, or a specific vocabularies used in the real situation beside the learner may also be able to understand the tradition used by the native.
