My suspicion, especially in the case of second languages, is that the "best" acquisition methods vary based on the person learning the language. Unfortunately, subjects initially learning their first languages aren't very good at discussing the details of what helps them the most.
Assuming we're dealing with speaking the language, interesting or not, some basic phonetics is needed. If you can't say the words in at least a partially understandable fashion, no one will understand you even if your vocabulary and grammar are perfect.
From phonetics, so many courses go off the rails. They focus on highly unlikely conversations, so end up using vocabulary that's of little use in daily life. Quite a few courses also decide to waste time on trying to hammer in perfect grammar. Language is about communication. Without adequate vocabulary, grammar doesn't really serve much of a purpose. "I doctor need" is grammatically incorrect, but gets the very important point across.
Here's one clue - even with different learning styles, what "specialized vocabulary" in your own language do you learn faster (assuming the same amount of time spent learning)? Specialized vocabulary for something you find interesting or specialized vocabulary for something you consider to be irelevant and useless? If you don't think specialized vocabulary is nearly qualified as another language, go listen to 2 lawyers discussing the legal theories behind tort law. Then go listen to an epidemiologist evaluating a research paper on the effectiveness of a new medication. Then go out and listen to a pair of aerospace engineers discussing how to modify a wing design to decrease drag and increase lift. All of the are using English grammar, but the vocabulary will be very confusing to the average layman. If you think about it, many hobbies have similar levels of specialized vocabulary.
For me, I'm plotting to continue sampling various Chinese courses, but may eventually have to design my own.
In an EOSL class I once took, I saw something that could be a great model for language and even grammar acquisition. It was a very short story. Only 2-4 paragraphs. Enough of the words were English to get started, but then there were . . . other words. It was designed so that the reader could contextually figure out nearly all the words and even some of the grammar quirks (in this case, plural was done by repeating the noun).
This sort of learning in context is how many people learn new words in their own language, so could be applied to learning a foreign language.