D'karr
Adventurer
Okay, technically Bards in PHB 2 cast spells and thus Vicious Mockery is an actual spell. Spells, as far as I can tell, are a specific thing in 4e with a formula (which can be written in a spellbook, at least in the description of spells for the Wizard). I'm going to go ahead and say this is your interpretation of the rules for Bards... but not necessarily the default for the game, or even a correct one.
Actually this would seem to be more like your interpretation for spells, as Bards do not require books and formula or to write spells in books.
Sorcerers, also cast "spells" and none of their spells fit that description either.