If “Getsu Fuhma” is destroyed by damage calculation, its effect still activates as long as it battled a monster and damage calculation was performed.
If an attack is negated by “Magic Cylinder”, etc., then “Getsu Fuhma”’s effect does not activate.
If “Getsu Fuhma”’s ATK is large enough to destroy “Vampire Lord” by damage calculation, then “Getsu Fuhma”’s effect doesn’t activate and “Vampire Lord” won’t be Special Summoned, because it was destroyed by damage calculation and not by a card effect.
If “Getsu Fuhma” is destroyed by “Dark Ruler Ha Des”, “Getsu Fuhma”’s effect is not activated.