While you are not wrong that high DEF is usually preferable over a mere 200 more HP (+15% of 200 is still only +30), it is Dragoon and Lancer specialized armor.
Dragoons and Lancers are all DPS units, which doesn’t have high enough base DEF to make +20 anything significant. Not to mention, nowadays there are DEF piercing capability on many new units, which render DEF useless. So, go for VIT.
Does anyone know if Aranea will be able to equip this. She is a dragoon/Lancer type.