Metamagic and vulnerability

Hello, i'm playing a sorcerer with the metamagic careful spell. Another player has an armor which make him vulnerable to fire. When my sorcerer cast a careful fireball on him, does he take damage or not ?

(Sorry for my english, i'm french)

"Vulnerability" to a damage type in D&D means that when you take damage of that type, it gets doubled. It doesn't affect whether you take damage of that type or not, just how much you take when you do.

If you're using the 5e/2014 version of Careful Spell, all that does is give him an automatic success on the saving throw, which for Fireball means he takes half damage. This would then get doubled by his vulnerability, back up to full damage. (If the original damage is an odd number it'll technically be off by one.)

If you're using the 5.5e/2024 version of Careful Spell, that version gives an automatic success on the save and says he'll take no damage if he would normally take half damage on a successful save. In this case, he takes no damage and the vulnerability is irrelevant.

When you cast a spell that forces other creatures to make a saving throw, you can protect some of those creatures from the spell’s full force. To do so, spend 1 Sorcery Point and choose a number of those creatures up to your Charisma modifier (minimum of one creature). A chosen creature automatically succeeds on its saving throw against the spell, and it takes no damage if it would normally take half damage on a successful save.

 So, if you cast a careful fireball, he makes his save, and, because fireball is a spell that does half damage on a save, he takes no damage.

All Vulnerability does is double the damage you take; it doesn't affect your saving throws. Since there's no damage to take, there's no damage to double.

That's make sense, thank you !

But it says: "A chosen creature automatically succeeds on its saving throw against the spell, and it takes no damage if it would normally take half damage on a successful save."

A vulnerable character take full damage on a successful save, not half damage. I'm confuses

Quote from Madmartigan54 >>

But it says: "A chosen creature automatically succeeds on its saving throw against the spell, and it takes no damage if it would normally take half damage on a successful save."

A vulnerable character take full damage on a successful save, not half damage. I'm confuses

You figure out the unmodified damage of whatever's happening before applying vulnerability.

For example, say the spell does 20 damage on a failed save and 10 on success. Those who are affected by Careful Spell would take 20 on failure and 0 on success. After that number is determined, you apply things like resistance and vulnerability. This means that with both Careful Spell and Vulnerability, they would take 40 (20 * 2) on failure, and 0 (0 * 2) on success.

Quote from Madmartigan54 >>

But it says: "A chosen creature automatically succeeds on its saving throw against the spell, and it takes no damage if it would normally take half damage on a successful save."

A vulnerable character take full damage on a successful save, not half damage. I'm confuses

 It's not how much damage you'd take -- it's about how the spell works. Fireball says you take half damage on a successful save. Therefore, it's in the class of spells that careful spell lets you avoid completely.

This example is from the rule in the PHB:

Order of Application

Modifiers to damage are applied in the following order: adjustments such as bonuses, penalties, or multipliers are applied first; Resistance is applied second; and Vulnerability is applied third.

For example, a creature has Resistance to all damage and Vulnerability to Fire damage, and it’s within a magical aura that reduces all damage by 5. If it takes 28 Fire damage, the damage is first reduced by 5 (to 23), then halved for the creature’s Resistance (and rounded down to 11), then doubled for its Vulnerability (to 22).

@Madmartigan54. Not sure if this would help, but you can reuse the example so Careful Spell would replace "a magical aura that reduces all damage by 5" with a protective effect that causes chosen creatures to automatically succeed on their saving throw, and they take no damage if they would normally take half damage on a successful save, so the Resistance and Vulnerability steps would not come into play for those creatures in your example.

EDIT: for clarity.