I have had interesting experiences using a prayer to the moon goddess Selene from an ancients book actually called the vampire bible.
Lilith is a powerful goddess who has helped with vampyrism in my path, as well as Naamah. If you call them and are serious, they will definitely help you in your vampyrism in my experience. They will lead you to books, teachings, outer body journeys and such.
Astarte has also helped me with this as well, including giving me specific instructions in my dreams of energy work and killing someone who was attacking us astrally or in the dream itself.
As far as a sort of astral immortality is concerned: doing a chi gung meditation called the golden flower and sticking with it grants this if you choose as well as many other abilities, but has the side effect of transmuting the dark side of your soul and adding it to your higher self. If done, you wonāt need to, nor will you really want to vamp others anymore as the darker aspects of your soul gets transmuted.
There is also a magician called Justin Bedwell that has a blog where he has given his version of a philosopherās stone glyph. This glyph specifically grants you astral immortality while also granting enhanced health, balance of elements, and many other abilities over time. It does this by refining the elements of your being by raising the vibration as to be impervious to astral attacks against it. It specifically carries the vibration of astral immortality and as you work with it and place it within your being, you keep it charged and self sustaining after a bit. But, again in my experience, it has also the side effect of taking away my needing or wanting to vampirize others and my dreams of Demi-Lichhood!
Fair trade for me!
Another magician called V.K. Jehannum has a blog and he is heavily into the vampyric current and mentions beings called the Vomquiserron who are supposedly undead gods that can guide the witch/sorcerer in vampyrism, as well as grant empowerment of many things astrally. Also he has an infernal vampiric rite calling upon qliphothic forces to help with this aspect as well. He also mentions his experiences with many goetic spirits that help with vampyrism as well.
Bardon has many spirits in his grimoire as well that grant astral immortality as well as a whole slew of powers, but according to him and my own experiences, they are much harder to reach and donāt easily give out their secrets to just any magician.
They will test you to see if you are really worthy of these powers, can handle the responsibility of them, and will not cause too much chaos with them.
From what I know of the Ardeth path, youād probably have a hard time getting them to help you terrorize others so that you can vamp off of them