I think that vocabulary matters much more than grammar to use a language. In many cases a learner's message with some grammatical mistakes could be understood. But without vocabulary knowledge it's impossible to express one's thoughts and to understand a message. Often it is not possible to guess vocabulary meanings in context correctly and precisely. The more vocabulary a learner knows how to use correctly the easier it is to convey a thought in a language in speaking, in writing and to understand the reading and listening content. Please read my posts here on English vocabulary issues.