Also, another good way, if you’re talking about increasing your vocabulary in English, and some other languages derived in part from Greek and Latin, is to try to familiarize yourself with Greek and Latin roots, prefixes and suffixes. There are a lot of lists of them available on the internet. These links are just to just one of them, not necessarily the best, just one of the first I came across in a quick search.
Common Prefixes
Common Roots
Common Sufixes
Becoming familiar with Greek and Latin roots, prefixes and suffixes won’t necessarily help you increase your vocabulary per se, at least not right away, but it will help you with the big picture, I guess; help you figure out what words that you come across and aren’t familiar with might mean and how they all kind of fit together in the grand scheme of the language and that, I think, will help you increase your vocabulary.