I had never heard the term before seeing it the other morning in my daily Merriam-Webster Word of the Day feed. Here's how they define "vade mecum:"
"Vade mecum" is Latin for "go with me" (it derives from the Latin verb "vadere," meaning "to go"). In English, "vade mecum" has been used (since at least 1629) of manuals or guidebooks sufficiently compact to be carried in a deep pocket. But from the beginning, it has also been used for such constant companions as gold, medications, and memorized gems of wisdom.
It's probable that not so many people carry around any sort of pocket manual any more. Even doctors no longer tote their 85-lb volume of the Physician's Desk Reference, which lists vital information for physicians: medications, their recommended dosages and possible interactions, and tee times at leading golf clubs. All this information is now an app; just buy it at the App Store and download it on your phone. Very handy. Same thing for other handbooks and manuals. Even the Bible is available to download.
In its other meaning, we all have Vade Mecum in the sense of stuff we Don't Leave Home Without. Wallet, keys, pocket change, pocket knife, bandanna, cell phone: that's mine, everyone has their own, and here's a big shout out to the person who invented pockets! Because we don't have but two hands!