Did you try to create a vorpX desktop shortcut? Shortcuts in many cases can help in problematic cases. Probably won’t help with the Steam version (Slender: The Arrival), but for Slender: The Eight Pages this *should* ensure succesful injection.
To create a vorpX desktop shortcut:
1. Right click the vorpX tray icon
2. Select “Create Desktop Shortcut”
3. Navigate to the game’s .exe in the dialog that opens
4. double click the .exe
This creates a special shortcut on your desktop that in some cases can hook more reliable than the auto detection.