AmorAstrum
[09] Warrior
My personal theory, is also that Viola is Amy, and like ArmorAstrum said, we don't really know if Amy is a vampire or not. Perhaps the infected blood on bare skin could have seeped through (Its possible, it is evil after all), then it may have took years to properly infect her, giving her immortality at a later age than when she originally received the blood. It is also possible that she never achieved immortality, and instead her aging slowed considerably.
I think it's a very plausible possibility considering that Soul Edge seems to affect each person differently when it comes into contact with them. Some people managed to stop aging, some people managed to turn into things that were no longer human, and some people manage to remain unharmed and untouched by it. It's really interesting to think about all the possibilities that it could take.