Vacuous is an adjective meaning “without contents; empty”. Its more specific meanings include “devoid of meaning”, “devoid of expression”, “lacking in ideas or intelligence”, or “idle”.
It comes from the Latin word vacuus, meaning “empty, void, free”. So yes, as you already know, it’s related to the word vacuum. But did you know that it’s also related to the word vain through the Latin word vacare, meaning “to be empty”? Yup.
Well, turns out there aren’t really any other interesting facts about vacuous – not that it’s a vacuous word, though. If you want to get a headache, just read Wikipedia’s article on the abstract concept of vacuous truth.