The Vuforia Plugin for Unity on Windows is a 32-bit native Unity plugin and requires the Unity 32-bit Editor to work. You can read more about native Unity 5.0 plugins here:
If running on Windows 64-bit, you will need to download both the Unity Installer and the Unity Editor 32-bit. If running on Windows 32-bit, you only need to download the Unity Installer.
Run the Unity Editor 32-bit Setup program first:
Run the Unity Installer second and uncheck the Unity component option* and check the Android Build Support component option:
* It is necessary to uncheck the Unity component option when running on Windows 64-bit OS, since the installer will attempt to install the 64-bit version of the Unity Editor if it detects that Windows is 64-bit. This would overwrite the 32-bit version of the Unity Editor you already installed.