Studying grammar is important if you are an adult. Children can learn it without studying, but adults usually lose that ability. That said, vocabulary is FAR more important than grammar in making yourself understood and understanding others. Look at these two sentences:
a) The xix fofs the ziz.
b) Boy like girl.
Both have the same meaning: The boy likes the girl. The first has vocabulary errors, but perfect grammar. The second has grammatical errors, but the key words are there. So there is no harm in studying both, but it is better to focus on vocabulary.