Exactly that happens — a vial of acid falls from the sky
A DM does not need game mechanics to describe the environment. When a vial of acid falls from the sky, the DM says "A vial of acid falls from the sky. Boom! It shatters to hundreds of glass pieces, splattering the surroundings with acid". That's all. You don't need rules nor rulings just for describing things.
Questions arise when we need adjudication. For instance, the vial of acid is a part of a trap, which a PC has triggered. Players don't want their character to be splattered with acid, so they expect some kind of roll or any kind of countermeasure, in order to feel the situation was fair:
«You hear a subtle "click", it might be a trap! Roll a Dexterity saving throw... Success! A vial of acid falls from the chimney up above. Because of the height, it takes a couple of seconds, so you've managed to jump out right before it hits the ground.»
The DMG explicitly describes dice as a DM's tool, which she can use, but doesn't have to:
Dice are neutral arbiters. They can determine the outcome of an action without assigning any motivation to the DM and without playing favorites. The extent to which you use them is entirely up to you.
Ignoring the dice
One approach i s to use dice a s rarely a s possible. Some DMs use them only during combat, and determine success or failure as they like in other situations. With this approach, the DM decides whether an action or a plan succeeds or fails based on how well the players make their case, how thorough or creative they are, or other factors.
The 3rd edition of D&D was trying to address every possible case in the rules, but apparently this didn't work well. The 5e returns to the "rulings over rules" principle from the 2nd edition.
The rules don't describe everything. It's just a toolset, which a DM can (but not have to) use for making adjudication. The DMG explicitly expects this from the DM (Part 3, Master of Rules):
The rules don’t account for every possible situation that might arise during a typical D&D session. For example, a player might want his or her character to hurl a brazier full of hot coals into a monster’s face. How you determine the outcome of this action is up to you
Tasha's Cauldron of Everything stresses this once more (see page 4, "The DM Adjudicates the Rules"):
[...] the rules can't cover everything. When you encounter something that the rules don't cover or if you're unsure how to interpret a rule, the DM decides how to proceed
So how do we figure out the outcome?
5e is not a physics simulator. Playing D&D is "an exercise in collaborative creation", as the PHB says. "A player triggers a trap with an acid vial attached" and "a vial falls from the sky somewhere" are two very different situations, which require different adjudications. The method of determining the outcome depends on what the situation is:
- If it just happens somewhere, simply describe it to players. Don't waste everybody's time on game mechanics when nobody needs them. If it hurts a NPC, just deal damage and move on. The game designers also suggest to "go with what's best for your story", if you're hesitant about the particular outcome.
- If it happens with players' characters and can potentially harm them, let the dice do their thing. Don't say "you take 30 acid damage", give them a saving throw. Basically you can use rules for traps, or just make it on the fly. A DM shouldn't browse the rule books in the process of the game.
- If it's a PC who drops the acid, but this works only in this particular isolated case, act in player's favor. In this case, you reward creativity. For instance, there is a fight in a lab, and a PC shoves some vials from the upper floor's shelves. The villain takes damage, but the shelves are empty now and the vials are shattered, so that trick can't be repeated anymore.
- If it can be easily reproduced, allow it but don't make it more effective than "traditional" means of fighting. For instance, a player asks their familiar to take the vial and drop it from 30ft on an enemy. If you says it makes more damage and hits automatically, the player would probably do this every time in every fight possible. This hurts creativity and contributes nothing to the story, such things shouldn't be encouraged.