As for death, I don't know if he became a vampire like Serio said... if so, he never died either. If not, it could be another process. It could be a spell, or anything else that doesn't involve death.
Well Rinaldo said that the crimson stone has a side effect to its usual vampire soul absorbing, and that is, the user of it will also become a vampire once he absorbs a vampire soul as far as I remember.
And the whole egg animation that appears once walter's soul is extracted and offered to Mathias, as stupid as it looked was most likely supposed to be a symbol, possibly of Mathias's change into a vampire, the beginning of his vampirism. Then he immediately started acting as one, fearing of the light, changing into a bat etc.
1. The bite
2. Illness (a few days)
3. Severe loss of blood, constant tiredness (a few weeks)
4. Death
5. Awakening (some period after death)
The cv one doesn't seem to be like that though. But then again, it was varying from game to game too. In Loi Sara's been getting vampiric impulses very quickly, but for example the priest in por seemed to be only tired but otherwise unaffected after roughly the same amount of time, and the twins took well over a year and were still not fully transformed, yet already had lots of power and didn't seem anywhere near dying.
And I'd assume once the infected one dies, it's impossible to revert them since they're just reanimated corpses once they reawaken. If that was the case, healing the sisters would kill them on the spot.