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? 英語(えいご)を勉強(べんきょう)して５年(ごねん)になりますが、まだ語彙力(ごいりょく)に(or が)乏(とぼ)しいです。 ＝I have been studying English for 5 years now, but I still have a weak vocabulary.