WotC Q&A said:Q: What happens when damage is dealt to a creature with both insubstantial and vulnerability? For example, let’s say I deal 1 radiant damage to a specter (who has vulnerability 5 radiant). What happens?
A: You apply the vulnerability first, bringing the total up to 6 damage, then apply the insubstantial, cutting it by half for a final total of 3 damage. You would do the same with resistance, so a specter taking 12 necrotic damage first reduces it to 2, which then gets cut in half to 1 because it’s insubstantial.
I was doing it the other way (1/2 dmg + vulnerable = total), much to my player's delight. I predict several frowns in the future thanks to this thread.![]()

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.