VLC generally prohibits setting the same hotkey for multiple functions.
In the example you gave of using the same key for
Exit fullscreen, the key for
Fullscreen already works as the key for
Exit fullscreen when the interface is in fullscreen. For other key combinations which obviously go together (like
Play/Pause), there are combined key versions in VLC's preferences.
If you try to set the same hotkey for multiple functions in the interface, VLC will warn you (Warning: this key or combination is already assigned to "Other function"), and then if you still go ahead it will remove the hotkey from the function it was used for previously.
Even when editing the preferences file manually to have two functions with the same hotkey, VLC has a priority order for functions, so it will only execute one function. In my experimentation, I assigned
Next to the same hotkey, but the hotkey always executed
If you really want to bind a set of unrelated functions together, you can use AutoHotkey for that (comment for more details).