If a thethic monster is reduced to 0 HP by a Mace of Disruption, what happens

A Mace of Disruption can kill a vampire outright.

You've already quoted and stated the most relevant details.

The Mace of Disruption says:

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.

While the vampire's Misty Escape ability states:

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.

The Mace of Disruption's effect doesn't simply drop a creature to 0 HP, it flat-out destroys them if they have 25 hit points or fewer after taking the damage and fail their wisdom saving throw.

Therefore even if the damage from the attack drops the vampire to 0 HP and it transforms into a cloud of mist it's still destroyed since the vampire's mist form doesn't grant it any kind of immunity to magical effects or being destroyed.

This GM has always read spells that bring back the dead as unusable for bringing destroyed undead creatures back from destruction. That is, undead creatures typically just can't be brought back from destruction as the same undead creatures they were before they were destroyed by using spells that bring back from the dead regular, everyday dead creatures. (D&D 3.5e, for example, has the spell revive undead (Spell Compendium 175–6) specifically for bringing an undead creature back from destruction as the same undead creature.)

When both the spells resurrection and true resurrection say, "You can revive… someone who has been turned into an undead creature and then destroyed," this GM has always played that the destroyed undead creature is brought back from the dead as the original and now-not-an-undead creature. (Of course, the soul may refuse to be brought back; see Special Spell Effects on Bringing Back the Dead.)

(As an aside, even other spells like raise dead and reincarnate may be able to bring back from the dead a destroyed undead creature as the original creature. For reasons I've never understood, these other spells say, "A creature who has been turned into an undead creature or killed by a death effect can’t be raised by this spell," yet they omit mention of the possibility of first destroying the undead creature. I don't know whether this is an accidental omission or intentional lessening of their power. Seriously, in this GM's opinion, typically, for every destroyed undead creature there's now a soul in the afterlife, and this is just an oversight, but ask your GM to be sure.)