com.android.vending is the package name of Google Play Store.
However, in Android, the apps installed outside from Google Play (from apk file) are installed through another system app embedded in every android phone. It is called Package Installer. com.google.android.packageinstaller represents the Package Installer system app, so you can completely say that users caught with com.google.android.packageinstaller are those who installed the app from an apk file, downloaded from anywhere else.