A vampire is notoriously hard to kill, thanks to its Misty Step ability:
When it drops to 0 hit points outside its Resting place, the vampire transforms into a cloud of mist (as in the Shapechanger trait) instead of Falling Unconscious, provided that it isn't in sunlight or running water. If it can't transform, it is destroyed.
While it has 0 hit points in mist form, it can't revert to its vampire form, and it must reach its Resting place within 2 hours or be destroyed. Once in its Resting place, it reverts to its vampire form. It is then Paralyzed until it regains at least 1 hit point. After spending 1 hour in its Resting place with 0 hit points, it regains 1 hit point.
So a vampire transforms to mist when it drops to 0 hit points outside its Resting place.
The Mace of Disruption has a special ability against Undead, which includes Vampires:
When you hit a fiend or an Undead with this Magic Weapon, that creature takes an extra 2d6 radiant damage. If the target has 25 hit points or fewer after taking this damage, it must succeed on a DC 15 Wisdom saving throw or be destroyed. On a successful save, the creature becomes Frightened of you until the end of your next turn.
The text here doesn't specify that a creature "drops to 0 hitpoints"; it just says "destroyed". Does that mean the Mace of Disruption overrules the Vampire's "drops to 0 hitpoints", due to the general 'specificity' rule of more specific rules taking precedence over general rules?
In short, can a Mace of Disruption kill a Vampire if the Vampire is at or below 25 hit points?*
* Assuming the vampire is out of Legendary Resistances for the day.