Vortex cloak is refueled by 5% for a single nightmarefuel, this represents 22.5 health of armor. Because of how armor works, no matter how much damage a player would take in a single hit, so long as armor has more than 0 health, it will block as if it had enough hp, so for vortex cloak, even with 22.5 hp it will block infinite damage once before going to 0%. However, if refueled by an odd number of nightmare fuel (leaving it with a decimal number) it will actually block 2 attacks regardless of how much damage the player receives, the first attack bringing it down to 1% (which is actually 0.5 hp) and the second clearing the decimal to actually 0 but will show -0%. This most likely has to do with a previously "fixed" bug where the vortex cloak would permanently get stuck at 1% (which was also it getting stuck at .5 hp in reality) but would never break as it didn't clear the decimal and thus was an infinite health armor vs most enemies.
As it is currently programmed, if the player refuels with 2 nightmare fuel (45 hp) of fuel (or any even number of fuel), then 1 large attack will properly take the cloak from 10% to 0%. This makes the item un-intuitive and punishes players for playing safe just because they don't know the bug that makes vortex cloak actually good.
Having reported this bug though, I genuinely feel that this makes the item actually worthwhile to use where as the intended behavior of the armor makes it under-preform compared to nearly any comparable chest slot item (while the stagger immunity is cute, it's strictly better to carry night armor inside a krampus sack or piggyback and for stagger immunity use thulucite crown's bubble which is more than enough for any situation). Leaving your armor at 5% to maximize the blocks per nightmare fuel is a high risk high reward strategy; can you afford to refuel your cloak in the middle of frantic combat and run around with only 2 attacks worth of block after which you'll be completely vulnerable? 1 attack worth of block for the 1 fuel high risk strategy (if the item functioned as intended) just doesn't have enough reward for the risk. So this bug unintentionally makes the vortex cloak a fun, powerful, yet risky item to use that rewards the player for playing well. I hope that you consider programming in an intentional 1% "safety net" for the vortex cloak as part of the solution. Such that, regardless of damage received, so long as the cloak has more than 1% of durability when attacked, it will then be reduced to 1%, if the cloak has 1% then it will block and be set to 0%. Or if you prefer (since %s are what's just shown to the player)...
if (attackDamageRecieved > cloakHp && cloakHp >0 && cloakHp !=1) then cloakHp = 1
elseif (cloakHp ==1) then cloakHp = 0
else (normal armor stuff)
1. Spawn in vortex cloak 2. Take attacks until armor is at 0% 3. Refuel with 1 nightmare fuel (works with any odd number of fuel as this leaves the cloak with a XX.5 value of HP) 4. Get attacked by something with more than 20 damage (such as a treeguard or 40 gunpowder). * observe that the cloak is now at 1% 5. Get attacked again *observe that the 1% blocks the full attack and the armor is now at -0% 6. Refuel with 2 nightmare fuel (works with any even value of fuel as this leaves the cloak with a XX.0 value of HP) 7. Get attacked by something with more than 45 damage (such as treeguard or 40 gunpowder) * observe that the cloak is now at 0%