After every session on Verbal Ability, I am flanked by students asking me this very question and in these exact words.
On different occasions, I try explaining them in different ways. In simple words I should summarize my recommendation in this manner:
By diving. Deep. Really deep. Into a subject.
What does that have to do with vocabulary? Besides, how does one dive into a subject? And what does it mean, the word 'subject'?
Subject is any definable interest. Yes, there are things which aren't defined on occasions. Or at least, definitions exist but not in one's awareness. So, the starting point is defining as one thinks fit or right. Architecture is a well defined interest. 'Finding patterns' perhaps isn't but it could definitely be one.
Vocabulary (as in, words) doesn't exist in isolation. One doesn't speak in single words. Words are used as parts of sentences which in turn are nothing but contexts. And contexts are nothing but shaped by 'subjects'. Cricket is a subject and the context could be the India-England Test Series. There are words which are specific to Cricket and Cricket only. If you want to talk Cricket, most likely you are going to converse in words meant to understand and express Cricket as you understand and see it. Diving deep into Cricket would mean that you know what each word in the context of Cricket means.
I wonder what 'hitting in line' would mean to a person who doesn't watch Cricket. Or a 'leg glance'. Or the words 'steered towards third man'. And if an entrance-test is skewed towards Cricketing contexts, I am sure this candidate is going to find it rather uncomfortable negotiating the questions.
So, is diving into one subject sufficient? Not really. The more number of subjects one dives deep into, the broader his exposure to different words is. And after a while, the magic happens. Suddenly, the words which earlier seemed very specific to Architecture naturally seem to be a part of Cricket and the words specific to Cricket suddenly seem to be a part of Fashion and so on...
Is Vocabulary not a subject? If you 'study' it, yes it is. But if you are just trying to remember words and their meanings, probably it's akin to diving in a well without water. Most likely you'll land on a mixture of sand, pebbles and rocks!