“Vapid” is used to describe something flavorless, weak, flat. Many people confuse this word with “vacuous,” which describes things which are unintelligent, lacking serious content. A boring speech may be vapid even though it’s learned, and a lively speech may be vacuous even though it’s exciting. A dull person may be vapid, but it is not standard usage to refer to a person as vacuous—only their speech, thoughts, etc., can be so described. To avoid the most common error involving these words, just remember that something vapid isn’t stupid, it’s bland.