It's very simple. Basically both words are different. "単語" means "word", and "語彙" means "vocabulary".
For example, you could use those 2 words like the following sentences.
この単語(たんご)の意味(いみ)は何(なに or なん)ですか？ ＝What does this word mean?
＝I have been studying English for 5 years now, but I still have a weak vocabulary.