What enchantment level is Mace of Disruption considered as i have it improved and i met some vampires in firekraag dungeon which i couldn't harm with Mace. (+3 weapons worked)

I think it is +2. Originally it hit as a +5 weapon but Sikret modified every weapon to hit as the base attack modifier. I do not agree with this solution because the THAC0 bonus could be different then the power of the weapon based on my understanding and I specially do not agree in this special case because the description says: "Any such creature [mean: undead] hit by it not only feels the sting of weapon, but also has a chance of being simply blasted from existence." and this +2 enhancement is far not enough to able to hit some (most) of the undeads in IA. Of course I agree that with a +5 to hit and damage bonus this weapon would be way too powerful for the price you have to pay for it and it would be unbalanced especially because you can get it very early but either I did not touch it (that would have been the best) or make possible to upgrade it.

Bad for me then. I upgraded it as i though it will be nice to have and looks like its actually pretty useless as a lot of stuff seems to require at least +3 enchantment (even some simple Vampires which are not a problem at all to kill, can't be touched by this weapon..). Thanks for info.

BTW. How this mace could prove usefull against the lich (as SpellStorm mentioned) - don't they require at least +3 weapon?

No. Liches, even the most powerful ones, the Grave Liches, can be hurt by weapons of +2 enchantment. The Daystar sword and the Improved Mace of Disruption are both of +2 enchantment and they can hit an unprotected by Magical Weapons lich. The Improved Mace of Disruption was not with +5 bonuses in the vanilla game - it simply hit as a +5 weapon, but this would seriously disrupt the balance in IA - imagine Skeleton Grandlords being slain by a single hit with this weapon if they fail a save vs death.

Still, although the Improved Mace does not affect the more powerful Undead, like Skeleton Grandlords or powerful Vampires, it still works against Skeleton Lords, and is an early-game item which grants permanent Negative Plane Protection, which is very valuable.

I haven't seen yet Skeleton Grandlords nor i killed even Skeleton Lord (i think) but i find strange that Mace can't harm simple Vampire (they are just named 'Vampire' nothing else) and die very easily to other +3 weapons. I forgot Negative Plane Protection - yea this may be usefull - but i did not fight much yet with enemies which lvl drain

The Mace of Disruption has enchantment bonus +1.
The Improved Mace of Disruption has enchantment bonus +2 (once the Ilithium ore is added).

There have always been some standard Vampires that requires a +3 weapon to hit - it's a hangover from the vanilla game. Of course you probably notice it more in IA because you will have fewer high-enchantment weapons.

Liches are not affected by Insect Plague.

Since Permanent Neg. Plane Prot. is omitted in v6, maybe Impr. Mace of Distruption could damage undead which requires +3 weapons to hit...

Also what is the effective enchantment level of the Equalizer?

Oops, maybe your mace is not imporved? If it is not improved then it is only a +1 enchancement and therefore it does not affect vampires. In standard game it does because it is counted as a +4.