Once again, Thornmail lets me down because Vayne got Mercurial Scimitar...
This time I reach +500 armor as Malphite, Vayne doesn't have good lifesteal, and how much damage does she take from Thorns? ZILCH.
And before you say "Thorns aren't late-game", then at what point are they ever GOOD? You get them early-to-mid game, you don't have enough armor to make the thorns strong. Get them late, they get countered by MR, lifesteal, Death's Dance, and/or Phantom Dancer.
So WHEN IS IT a good time?
(Match history doesn't seem to be working right now, but we lost the match because Cho'gath D/C'd and didn't come back.)
EDIT: To address any misinterpretations, I was not expecting the 500 armor to help me SURVIVE Vayne's damage (nothing works against Vayne). I was, however, expecting that the Thorns damage would seriously hurt Vayne, but she ended up taking nothing from it. THAT is what's pissing me off.
the adc now has to rush
and 
because it sits at 103% gold efficiency, but if you don't need the mana is back to 80% and that's why only tanks with mana issues buys it, since only on them is gold efficiency is worth the time.
will yield better results than
ineffective against you because even after they reduce your armor, you'll still have a large amount left over that overpowers the ArPen effect (unless the math mysteriously doesn't work like that).
into your build or take some marks to help beef up the damage.