Can a familiar use a vial of acid?

Question

Can a familiar use a vial of acid or similar items? I'm wondering if they're able to since I am making a warlock with proficiency in Alchemist’s Supplies.

Answer 1

Only if you have a feature that permits the familiar to make an attack.

The description of a vial of acid says:

As an action, you can splash the contents of this vial onto a creature within 5 feet of you or throw the vial up to 20 feet, shattering it on impact. In either case, make a ranged attack against a creature or object, treating the acid as an improvised weapon.

So using a vial of acid involves making an attack.

However, the description of the find familiar spell states:

A familiar can't attack, but it can take other actions as normal.

But you said you were a warlock, which most likely means you are a Pact of the Chain warlock (since you have a familiar), and so you have this feature:

Additionally, when you take the Attack action, you can forgo one of your own attacks to allow your familiar to make one attack with its reaction.

Which would allow your familiar to throw the vial of acid using its reaction.

Otherwise, if you are not a Pact of the Chain warlock, and you acquired find familiar through other means (such as via the Magic Initiate feat), your familiar cannot attack and you have no features that allow it to do so.

Answer 2

Normal familiars can't

Throwing or splashing a vial of acid (same goes for alchemist's fire and holy water) requires you to make a "ranged attack against a creature or object".

A familiar from the find familiar normally can't do this, because "A familiar can't attack".

...but Pact of the Chain warlocks' familiars can

If you are a warlock with the Pact of the Chain feature, your familiar can attack:

Additionally, when you take the Attack action, you can forgo one of your own attacks to allow your familiar to make one attack with its reaction.

However it does still use your Attack, so the upgrade is limited to effective range. And possibly attack modifier, assuming your familiar is more dexterous (i.e. better at ranged attacks) than you. (The optimal familiar for this end is a sprite, as it has a Dex modifier of +4.)

Answer 3

RAW, no, but it need not be unbalancing to permit it

If by 'use a vial of acid' you mean make an attack with it, then no.

As stated in the spell itself, a found familiar can't attack:

A familiar can't attack, but it can take other actions as normal.

However, the familiars summoned by the Pact of the Chain Warlocks are an exception:

Additionally, when you take the Attack action, you can forgo one of your own attacks to allow your familiar to make one Attack of its own with its Reaction.

So Pact of the Chain warlock familiars can attack, but not by taking the Attack Action. Rather, they must spend their Reaction to do this. And, as pointed out by Exempt-Medic, one may not use a reaction to throw a vial of acid:

As an action, you can splash the contents of this vial onto a creature within 5 feet of you or throw the vial up to 20 feet, shattering it on impact.

Because the definition of special use for a vial of acid (or oil, or holy water, or Alchemist's fire) includes the requirement that it must be 'splashed or thrown' as an action, even your Chain familiar cannot effectively attack with it, since their attacks are permitted only as reactions. [Note that, as Thomas Markov points out in the comments, their status as a chain pact familiar does grant them the ability to attack with a reaction, per se. However, this attack would be limited to them using the acid vial as a generic object Improvised Weapon, for d4 damage. The special properties of the vial, splashing or breaking and doing 2d6 acid damage, can only be activated as an action, as specified in the description of the vial, and are thus unavailable to the familiar RAW]

Is it really so bad?

Suppose you are not satisfied with this rule. What would happen, worst case scenario, if you decided to change it?

If familiars can throw acid as a reaction, why not PCs?
A character typically attacks as a reaction on an opportunity attack. Opportunity attacks, by definition, are limited to melee attacks. All of these items (acid, oil, holy water, Alchemist's fire) are thrown Improvised Weapons making ranged attacks. But if a familiar is permitted to make a ranged attack as a reaction, why then couldn't a PC make a ranged opportunity attack? Given all of the options available to PC's for ranged attacks, this represents a fundamental change in the rules with the likelihood for some unbalanced consequences. I would advise against it unless you enjoy playtesting.

How about permitting familiars to take the Attack action?
Another way to allow familiars to attack with acid would be to permit them the Attack action rather than restricting them to attacking on a reaction. Care must be taken with this; in a game that revolves around the action economy, we cannot allow a single first level spell (find familiar) to give a character an additional Attack action each round, for every round the familiar participates in the combat - particularly when one can arm a familiar with magic items. This reasoning is behind the injunction on familiars attacking to begin with. Even Pact of the Chain warlocks can only have their familiars attack at the cost of (one) of their own attacks.

With the restriction in place that the controlling warlock needs to sacrifice an attack, what could go wrong with allowing a familiar the Attack action? Well, nothing unbalancing, but there is the great inconvenience that comes from their having a separate initiative from the warlock, as defined by the find familiar spell:

Your familiar acts independently of you, but it always obeys your commands. In combat, it rolls its own initiative and acts on its own turn.

Having the familiar use a reaction to attack is what allows the attack to happen on the warlock's turn, on the same turn that they sacrificed their own attack, thus keeping everything logistically tidy. If instead there is temporal separation between the warlock's sacrificed action and the familiar's subsequent attack, it is going to get messy. At best it becomes one more thing to keep track of, remembering from the warlock's turn until the familiar's turn that an attack was sacrificed and another one enabled. At worst the familiar will die or become incapacitated before its turn comes up, making the warlock's player feel cheated and unlikely to use the ability again. Or the player will seek to exploit the situation into a readied action with a sloppily-declared trigger, passing their Attack to the familiar with full freedom to decide what to do after something else has resolved, treating it as a delayed or deferred action, which is not a thing in 5e and for good reasons.

My recommendation

If you really want to permit a familiar to throw acid, without running afoul of the action economy, opening the door to player exploits, or creating logistical entanglements with initiative, I would recommend wording the Pact of the Chain warlock's ability as follows:

You learn the Find Familiar spell and can cast it as a ritual. The spell doesn’t count against your number of Spells known.

When you cast the spell, you can choose one of the normal forms for your familiar or one of the following Special forms: imp, Pseudodragon, Quasit, or Sprite. In addition, any of the familiars you summon act on your initiative and share your turn.

Additionally, when you take the Attack action, you can forgo one of your own attacks to allow your familiar to make one Attack of its own by granting it its own Attack action.

Moving the familiar's initiative to yours is similar to what was done with the Artificer's homunculus, which has the ability to deliver a touch spell cast by the Artificer. Consolidating the familiar's turn to yours further removes the confusion that can arise from operating on different turns. Such a change should allow your familiars to throw acid and other ranged improvised weapons with the least disruption to the rest of the rules structures of the game.