I did more tests since last time and I figured out that what I did for combining ARCore sdk and Vuforia 7.2 in only one Unity Project does not work on Android 8.0 devices.
If you check ARCore supported and Vuforia supported in Player/XR Settings, on an Android 8 devices, you will have a "Vuforia Initialization Error". However, if you build a first time with only Vuforia supported checked, launch the app, and then rebuild with both checked, you won't have the Init Error and everything will work as expected. I think this is because the initialization works the first time (without "ARCore supported" checked), and store on the device an information telling "initialization ok", then when we build&run a second time, this time with "ARCore supported" checked in addition to "Vuforia supported", the app retrieves this information on the device and the initialization works. But if you remove the app from your smartphone, and build&run again with both ARCore and Vuforia supported, you will have again the initialization error from Vuforia.
Unfortunately this workaround cannot be used with final users because only the dev can build a first time with only Vuforia supported and a second time with both ARCore and Vuforia.
So the behaviour is not the same on Android 7 and Android 8 (on Android 7, it works with ARCore and Vuforia supported even at the first installation just by changing the AndroidManifest.xml as I specified in a previous post).
EDIT : Finally, I found a solution !
It was an error with permissions so I checked AndroidManifest.xml generated by unity in Your_Unity_Project/Temp/StagingArea folder.
And I found this strange line : <meta-data android:name="unityplayer.SkipPermissionsDialog" android:value="true" />
Unity add this line when you check ARCore supported, I don't know why. Because of this, the user couldn't allow the app to use camera, resulting in Vuforia init error.
To fix it, add the line above in your AndroidManifest.xml, and change android:value="true" by android:value="false", then it will work with Vuforia and ARCore both checked in XR Settings for Android 8.0 !