Hi, today i had the same problem(with vuforia 6.2.10 and unity 5.6.1) but now i got my app working. So i will tell you what i did hoping that help you to make your app working aswell.
i tested on 2 devices and got the results below:
Asus Zenfone Go: app was not recognizing the images
Motorola Z Power: got the message "Your app key is missing"
1 - Deleted every single file/folder of vuforia package from my project. Trying to reimport the assets you can see the files and folders so you can delete them without any problem. you will get some missing stuff on the scene but you should be ok later, i recommend that you save your project before reimport vuforia package, close the unity and then reimport.
2 - Try development build and see if it is working, if not, stay in development build until it works with the next steps.
3 - Player settings: Here was the tricky part for me(i think).
a: unchecked split application binary option at Publishing Settings
b: i was exporting the same version, replacing the apk file and getting the problem every time, change your version and bundle version code, the name of the apk file will be changed automatically.
this is what i did to make it work again. Good luck trying, i hope that it helps you.
PS: Still learning english.