Technically speaking, to avoid the second death, the death of the astral body, you need to sustain it with what compose it : energy/life essence. It the current of vampirism this is done by extracting this essence from living beings.
Regarding the crystalization process. The human beings are generally described to have a physical body, a mental body, an energetic and astral bodies, and one or multiple “superior bodies” that could be assimilated to the western concept of soul.
That soul is the link to the creation/God/The cycle of life/death/and rebirth that goes with it and is supposed to be “permanant”.
The crystalisation of the astral and mental bodies is done by cutting them from the superior body, and by the same occasion from God/the cycle, while it by a connection to something else : the vampiric strain/the essence of a vampiric mentor.
This create a new being