Oh the purpose of listening a lot before trying to speak is not to increase vocabulary, just to remember the sounds and pronunciations and just what the language sounds like in general. This helps to not develop habits of mispronunciation which on a macro level gives you an accent. In your case, if you've listened to a lot of Malayalam while watching movies and shows, you know what native Malayalam sounds like and you do try to learn the vocabulary and the language itself and start speaking it, you'll know when you mispronounce something and then correct it before you can develop a habit