Voadam
Legend
OK I just watched that and it says monster spells are still spells but magical abilities like a mind flayer's mind blast are magical but not spells.I know that Crawford clarified on Twitter somewhere, though I believe that he goes into it a bit here:
That seems to support Vecna's explicit spells under spellcasting being spells and his other magical abilities being not spells even if they act a lot like spells.
Same as it has been throughout 5e.