i think three of them are very important!without vocabulary u will have a difficulty to speak or express what u want to say...pronounciation is very important too..if u cant pronounce the english vocab properly people will get confuse of what u trying to say...but grammar not very important but dun be too poor coz it can make the person u talking with confuse..^^

you talk about learning the language, therefore i do believe all those three you've mentioned  are important. first things first, basic vocabulary, then learn how to pronouns that basic vocabulary that you've learned, then construct simple sentences out of that basic vocabulary then engage in a simple conversation to enhance your skills in speaking.

I'd look to your question as it is precised, because they're all necessary in language learning, but have different dergee in their value.
I think the most important to master the language is vocab, to have knowledge of the exact meanings of the language's words is what makes you able to go deeper and be good at it.
The other important thing for who already masters the language is pronounciation. You'll be "too good not to be true" with an excellent vocab and a broken pronouciation.. By the way, what makes us differenciate between a native and a fluent foreigner? It is pronouciation. Even if you're not too good in the language, yuor pronouciation will cover on that (at some points) :-p

Al Salam, 

if we are talking about communications or asking about something....
as a new forign language, its important to know the basic vocabularies (directions, places, ...etc) and to know how to pronounce it so the person u are talking about can understand u. i think in this case grammer came later.

on the other hand , learning a new language in an academic way, makes all of the three important at the same level...

I believe that at the beginer level grammar is the most importient. How can you make a sentense without knowing any grammar rules. But on the next step the most importien things are vocabulary and listening/speaking skills. And it's not a big problem if your speach will have some mistakes. The most importient is communication. But how can you say something with minimum of vocabulary??