If we focus on a learner centred approach, then perhaps we are looking at acquisition. As Scott Thornbury wrote “The learner should not be seen as the object of the verb “to teach”, but the subject of the verb “to learn”.
I have also researched this field and particularly read Bill Van Patten and David Crystal. A summary of what I gleaned is as follows.
Vocabulary is not just about meaning. In Spanish “pan” means “bread”, this is a meaning. However, the words “pan” or “bread” also have syntactic properties. The word “bread” is a noun, it is also countable. That is why we say “some bread” not “a bread”.
So, when we acquire language, not only do we learn the meaning, we also acquire the syntactic properties of the vocabulary. We learn the underlying concept of where and how a word can be used in a sentence. This is not too different from what is traditionally called “grammar”. Chomsky´s famous example of “colourless, green ideas sleep furiously”. Obviously, this phrase has no real meaning, however it is grammatically correct.
Crystal, as quoted from “Grammar should be renamed “understanding language”, in the Guardian, argues that grammar needs context, much like vocabulary. “You need to put the notion (of grammar) in the background. It´s about meaning and clarity”.
Another salient point is where does vocabulary end and grammar begin?
When teaching common collocations, such as dependent prepositions, do you consider them grammar or vocabulary?
Vocabulary and grammar are acquired in much the same way. Mental representation of language is both word meaning and properties. Input and interaction are key for language acquisition.