Can a vial of holy water be used as a weapon?

Question

In the Player’s Handbook the section for holy water states (emphasis mine):

As an action, you can splash the contents of this flask onto a creature within 5 feet of you or throw it up to 20 feet, shattering it on impact. In either case, make a ranged attack against a target creature, treating the holy water as an improvised weapon. If the target is a fiend or undead, it takes 2d6 radiant damage.

In both Curse of Strahd and Storm King’s Thunder characters can find vials of holy water, not flasks. These could be used as a material component for certain spells, but can they be used a weapon like a flask of holy water? If so, how would the damage differ?

The Player’s Handbook also defines the capacity of a vial as “4 ounces liquid” and a flask (or tankard) as “1 pint liquid”. So could a character combine four vials to use as a weapon?

Answer 1

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:

A vial can hold up to 4 ounces of liquid.

A flask holds 1 pint of liquid.

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:

[...] Bless Water. You touch one vial of water and cause it to become holy water.