Final answer:
The rule of vocabulary mentioned by Stephen is to use plain, direct words in your writing and avoid using words that sound fancy or impressive if you are unsure about their meaning.
Step-by-step explanation:
Stephen mentions a rule of vocabulary, which is to use plain, direct words in your writing and avoid reaching for a word that sounds fancy or impressive, especially if you are unsure about the meaning. Instead, use words that you are familiar with and understand. If you encounter a word that you are not sure about, look it up in a dictionary to ensure you are using it correctly. By following this rule, you can avoid confusing your readers and improve your writing skills.