Decreasing 5% upped? No way
I would rather lower the costs then change it from 10% to 5%. What leads me to my old question: What exactly does this Vitamin protect from? Is it always handled as Quinted?
For example: 4th turn (I played the card, creature on 100%, I end my turn, still 100%. My turn again, I end it, 90%. My turn again, I end it, 80% -> 4th turn, until I end the turn it's at 80%, right?), the creature is at 80% of protection (unupped; 80% then, right?). Is there an 80% chance that, if attacked by a Thunderbolt, the protected creature pops up some kind of *Quinted* like SoFr creatures when evading and the attack is negated?
In general the -each turn 10% less- is a good thing