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.