@JeremyECrawford Creature w/ Immunity to Fire is hit by Contagion spell & becomes "vulnerable to all damage." Is it now vulnerable to fire?

— Ed Robinson (@Lwnasidh) June 16, 2016

If a creature is immune to a damage type, making the creature vulnerable to that type does nothing (2 x 0 = 0). #DnD https://t.co/GV926kIYIu

— Jeremy Crawford (@JeremyECrawford) June 16, 2016

@JeremyECrawford tweet is better without the shorthand since you could also argue base % -100% + 100% = base%

— Justin Melillo (@Cybren) June 16, 2016

Vulnerability is applied after all other modifiers to damage (see PH, 197): 0 damage passed to vuln. stays 0. #DnD https://t.co/K4yOfEMzpe

— Jeremy Crawford (@JeremyECrawford) June 16, 2016

@Cybrenright but my point is people that already know that aren’t asking you! Indeed. Hence my response.

— Jeremy Crawford (@JeremyECrawford) June 16, 2016