語彙 and 単語: a vocabulary range and an individual word
Compare usable vocabulary expanded through reading with an unfamiliar word written in a notebook. Context-sensitive word choice is not established by a card count alone.
A range of words and vocabulary choice
語彙 (ごい) means vocabulary: the set or range of words used in a language, by a person, or in a field. 「読書を続けると使える語彙が少しずつ増えた。」 says continued reading gradually expanded usable vocabulary.
「文章に合う語彙を辞書なしで考えてみた。」 describes trying to think of words suited to the passage without a dictionary. The word can concern selecting suitable vocabulary as well as its overall range.
An individual word
単語 (たんご) means a word considered as an individual item. 「知らない単語をノートに書き留めた。」 says an unfamiliar word was written down in a notebook.
「同じ単語でも、文脈によって意味が変わることがある。」 says the same word can vary in meaning with context. It does not say that the meaning always changes.
The range and the use of an item
Expanding usable vocabulary and noting an unfamiliar word have different focuses. The former concerns a range of usable words; the latter concerns an individual item to examine.
The number of word cards does not establish the ability to use every word appropriately. Also examine suitable wording for a passage and the contextual meanings of the same word.
What kind of item is noted in 「知らない単語をノートに書き留めた。」?
Show the answer and why
An unfamiliar individual word. Noting it does not establish understanding all its meanings or having fully acquired it as usable vocabulary.