Mage hand can pour out a vial of acid.
Pouring a vial of acid in this way will (generally) not damage an enemy.
It's exactly as the rules you quoted say. You can pour out the contents of a vial. Per the description of Acid, simply pouring isn't enough to do someone--who likely doesn't want acid poured on them--damage.
If you start pouring acid on one who doesn't want to be acidified, they can move, interpose an object, or otherwise evade the damage. In order to damage someone with acid you've got to splash it onto them, an activity that's defined as a ranged attack. And Mage Hand, per its description, can't effect damage on someone trying to not be damaged: it can't attack.
Could one pour the vial over an unsuspecting enemy, or one incapacitated or restrained, thereby causing them damage? I contend it follows the same rules: if you would need to make an attack roll, Mage Hand can't do it. Whether those situations require an attack roll--and therefore are attacks which Mage Hand cannot effect--is a ruling to be made by the GM.
This spell is the only official mention of "Unholy Water"
A quick search on D&D beyond, the official D&D 5e web toolset, reveals that the Commune spell is the only official mention of the term "unholy water".
That means we have to fall back on natural language
Since the term has no official game meaning (because it is only mentioned once as a spell component and nowhere else), we have to use the natural language definition of the terms.
Unholy water is, obviously, water; it's in the name.
Dictionary.com defines unholy as:
1.
not holy; not sacred or hallowed.
2.
impious; sinful; wicked.
By this definition, unholy water could just be water the isn't holy water. However, this would mean that something like tap water could be used as the material component of the spell. Since, Commune specifically allows you to talk to a deity or divine proxy, I would argue that this non-religious/non-divine definition isn't what is intended.
So, based on the second definition, unholy water is some way the opposite of holy water. Where holy water is water that is blessed and carries a positive religious connotation, unholy water is profane, wicked water with a negative religious connotation.
RAW however, that means unholy water doesn't do anything
As you have noted, there are no rules for creating "unholy water". There are no descriptions of any mechanical effect that it has, or any uses for it besides this one spell. RAW, it is simply used for the Commune spell and nothing else.
It has no other properties RAW, and any other mechanical effect given to unholy water would be a DM call.
Best Answer
There are many items left undefined; they are left up to the GM
There are several items in the PHB which are left undefined, such as the Mess Kit, Grappling Hook, Hammer, and Chain; and others are added in Curse of Strahd such as garlic, salt, and vials of holy water. None of these are specifically defined in the rules so how they work would be left up to a GM. The only thing I could find that vials of holy water would explicitly do is work as a material component for spell which require amounts of holy water.
It is certainly reasonable to use the descriptions of vial and flask in determining these effects:
There are sixteen ounces in a pint, so there would be four vials in one flask. Perhaps then a vial deals one-fourth the damage of a flask.
Also note that those are not the only times vials of holy water exist, the ceremony spell states: