(At least "Vlc" works for me, but "vlc" doesn't (i guess xmonad only checks
one of the possible class names, or something)).
There is only one class name; you're being confused by a fossil in X11. (There are lots of these fossils; it's one of the big reasons everyone wants Wayland so badly. Large chunks of X11 still date from the 1980s, and new functionality has been duct-taped wherever it could be forced to fit.)
WM_NAME: window title
WM_CLASS: first value is appName, second is className