For example, I am vulnerable to fire and have an item that gives resistance to fire. Do the two cancel each other out, or does one supercede the other?
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\$\begingroup\$ Is your vulnerability a character trait or brought on by an item? \$\endgroup\$– Lino Frank CiaralliCommented Dec 29, 2017 at 21:48
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\$\begingroup\$ [Related] How do Damage Immunity and Vulnerability work together? \$\endgroup\$– SevenSidedDieCommented Dec 29, 2017 at 22:16
2 Answers
Resistance and then vulnerability are applied after all other modifiers to damage. (PHB 197)
This tells us that both are applied if present. In most cases this will mean that the damage taken is not changed, but technically there may be rounding "errors".
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6\$\begingroup\$ "Most cases" isn't exactly correct - all odd numbers of damage are reduced by one, so it'll be exactly half of the cases where damage is rolled that are unchanged. Minor nitpick, I know :) \$\endgroup\$– kviiriCommented Dec 29, 2017 at 21:45
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1\$\begingroup\$ twitter.com/jeremyecrawford/status/930576764148924416 you might want to incorporate this tweet into your answer. It's not necessary, but this is confirmation that the statuses stack as you describe, Rules As Intended. \$\endgroup\$– kviiriCommented Dec 30, 2017 at 17:51
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\$\begingroup\$ @kviiri Thanks, but he essentially says RTFM and does not clarify much except by omission (does not say otherwise). \$\endgroup\$– SzegaCommented Dec 30, 2017 at 18:22
No they don't, but you do get a 1hp bonus if the incoming damage is an odd amount.
"Whenever you divide a number in the game, round down if you end up with a fraction, even if the fraction is one-half or greater. This is the case in every circumstance unless a more specific rule (see below) specifies that you should round up instead. If any doubt emerges, round down!"
"Resistance and then vulnerability are applied after all other modifiers to damage."
So you have a tiefling with fire resistance, but a curse or effect that gives fire vulnerability. Now you take 25 incoming fire damage. This is first halved from resistance 25/2=12.5 rounded down to 12 and doubled back to 24 for the vulnerability. If the incoming damage was an even amount say 24, it would be halved to 12 then doubled back to 24.