Ytdyklly

When dealing with monsters who have resistance or vulnerability to a specific damage type, I've been having my players roll the expected damage and I'll half or double it behind the screen. I've recently found a DM who tells the players outright if the monster is resistant or vulnerable, and, given the example of vulnerability, has them roll double dice instead of doubling the damage.

Should the player roll damage and the DM secretly halve or double it depending on the situation of resistance/vulnerability? Or should the DM tell the player that the monster has resistance/vulnerability and have the player roll half/double the damage dice?

An example:

Player Bob is playing Bob the Barbarian. He hits a (DM-controlled) Grick with his club. The Grick is resistant to non-magical bludgeoning damage, and the weapon is non-magical.

Do I, as the DM, tell Bob the Barbarian before the damage is done that the Grick is resistant to non-magical bludgeoning damage and have him roll and halve the damage himself? Or do I hear his total damage roll, halve it behind the screen and explain that while the hit was good, the Grick doesn't appear to be as fazed by it as he's used to seeing monsters when he hits them this way?

(Does the player roll it [knowing ahead of the damage roll] or does the DM take it in and explain it through the story?)

There seem to be two questions at play here. First, whether to double the number rolled or the number of dice rolled, and second whether the DM should conceal vulnerabilities and resistances.

Should the dice result or number of dice rolled be doubled when attacking a vulnerable creature?

The Player's Handbook says, under "Damage Resistance and Vulnerability" (p. 197):

If a creature or an object has resistance to a damage type, damage of that type is halved against it. If a creature or an object has vulnerability to a damage type, damage of that type is doubled against it.

This would indicate to me that rolling the dice then doubling or halving the result is the expected way to do it. Rolling half or double the number of dice will have less variance and will trend toward a more average result, but it's still quite close and a DM choosing to do it the other way is perfectly reasonable.

Should a DM reveal whether a creature is vulnerable or resistant to a specific type of damage?

This question is a bit more subjective, but I would say yes, in most cases if a character hits the creature with a type of attack that they are vulnerable or resistant to the player should be informed that it was more or less effective than expected. A wizard who casts a fireball at a succubus should be able to see that it barely scorched the fiend's hide and be able to modify their tactics accordingly. Likewise, a fighter that hits a skeleton with a club should be able to see the bones crack and splinter under the force of the blow and see that it was effective. I would not suggest informing the players of vulnerabilities and weaknesses beforehand, as that is one of the big uses of knowledge skills, but when they use effective tactics their characters should be able to see the results firsthand.

In addition to what Kyle Doyle's answer covers, here's a quote of the rules regarding critical hits:

Roll all of the attack's damage dice twice and add them together. Then add any relevant modifiers as normal.

Compare that to the rules text regarding vulnerability:

If a creature or an object has vulnerability to a damage type, damage of that type is doubled against it.

To me, the difference in wording suggests that the intended implementation of the rules is to let the players calculate their damage dealt as normal, and then double that number to calculate the total damage received (or half it in case the target is resistant).

One thing to note is that rolling double the dice or doubling the damage dealt will yield different results, as the former does not double modifiers, whereas the latter does.

As to the question of whether and how to inform the players of resistances and/or vulnerabilities, I would agree that descriptive fluff is the best way to do so: describe that the fireball seems to affect the enemy more (or less, in the case of resistances) harshly than expected. Use of knowledge skill checks might inform players beforehand.

Resistance and Vulnerability damage takes place after all other modifiers are done. The damage would be done by the players FIRST, then the half or double would kick in.

I'll continue to do it the way I have been as it seems that's more correct.