35 Armor with a passive that deals damage to attackers and give them grievous wounds.Why would that be so strong? If we compare that to our AD or AP counterparts the AD has 15 ad and no unique passive for 200 gold less so it has less stats and no unique passive and to AP (Morelicom or whatever is spelled),you gain Grievous when you finish the item.So from all of them,the tank one is the strongest.
Well if you play as a tank against an AD Healer (Fiora,Irelia etc) it will win your lane easily since not only it grants armor which counters ad champions early game a lot but it deals damage to attackers which is not that much but the grievous wounds will lose the all ins against tanks a lot,which most of the bruisers with heal have to do to win lanes against a tank.
Now tanks are good in teamfights so why do they have an item to win easily the laning phase too?
So what do I suggest to be done is to remove the grievous wounds from Bramble Vest,yet the thornmail to keep it.
Most of the tanks build this item after proceed to never upgrade it,till the last item since they keep it for the Grievous Wounds most of the times.The item could receive a gold buff to 900 to compensate for that and thornmail cost to be reduced by 100.