If the Vahi or Ignika were destroyed outside of the Matoran universe, its destruction would have no effect, as it was keyed only to affect the Matoran universe and not the world beyond.
Masks were considered legendary because of the elements they controlled, not their rarity.
The Ignika and Vahi were on the same power level because the elements they represented were codependent.
The Ignika and Vahi didn't necessarily have different levels of power, just different kinds of powre. Life couldn't exist without time, and time was irrelevant without life to affect.
The Ignika and Vahi were equal in power as life couldn't exist without time and time without life was pointless because there would be nobody to mark its passing.
The Mask of Time and Mask of Life were legendary masks, but they were not considered elemental masks because time and life weren't Toa elements.
Legendary masks were more resilient and could take damage better than other masks.