To re-iterate what many people think, I have never seen an immortal vampire or any such life form. I'm very sure that it's impossible to sustain a human body by leeching energy from other individuals. In fact, there is a backlash to consciously draining energy off of another out of spite. Just don't do it.
The Astral Plane
Something you'll need to know is that the astral plane (or 4th dimension) is very closely tied to our physical plane and so most of what everyone does utilizes and is bound to either the physical or astral. The typical life cycle isn't an exception. A soul will jump from the astral into the physical and incarnate into a body in this plane. We see this as birth but what we don't often consider is that the soul has been existing for nearly as long as the universe. So really, it's only the birth of a body and the soul itself controls that body and experiences reality as a member of that species for a short period of time before dying and returning to the astral plane. Then the cycle restarts some time later and continues until some amount of personal growth is achieved. Unlike the physical which flows very slowly, things happen quite instantaneously on the astral. Things aren't as condensed there so it takes very little effort to move and materialize things. If you want to fly, you fly. If you want it to snow, it snows. If you're paranoid that a hit squad is going to open fire on you, well anything you think happens. Many will argue that dreams also transpire on the astral plane.
With that out of the way, let's get to feasible methods of gaining immortality, like the ancient Egyptian rites. They had an entire system or school in which students were taken through many lessons both experiencial and logical. This is to say that they didn't just study information and then manipulate their energy. They also learned through pure experience which you can't achieve by simply reading a book or hearing a lecture.
Students would learn all about energy and sacred geometry, and they would even have to re-acquaint themselves with their past lives and all aspects of their souls. Honestly, I think if we all did it their way today, we'd have much less confusion in society but that's a topic for another time. They were also taught how to "ankh" their sexual energy by having it jump out the back of the body at the heart chakra at the time of release and then looping that energy around their head and back into the front at that same chakra. They would then focus the flow of energy simultaneously from their head and feet into the heart chakra. During orgasm, they would move their energy around using what they knew about the human energy system and it would prevent them from losing any life energy. It even supercharged them.
Students were also trained to abolish their fear because in reality, fear could be harmful. They based this off of the astral plane and how it reacted to thoughts. They surmised that anyone prone to any amount of fear would get to the astral plane and then those fears would project out and destroy them at which point they would be forced out of the astral and back into the physical to start another life here. But they wanted to forego the process of reincarnation altogether. Students needed to basically master the physical and astral and get to the point at which they could nourish their bodies using pure energy. Regarding one Egyptian school, about 300 people were able to achieve immortality.
Note on Immortality
You may or may not have realized it yet but souls can't die. A physical body can die but the astral body and all of its memories are pretty permanent. The interesting complication that we've been having on Earth is souls incarnating into bodies and then never recovering their memories. This essentially makes them a clean slate every time and even if they incarnated to accomplish one particular thing, it's really a long shot as to whether they succeed in remembering it or just do it on accident. Because souls can't actually die, we can call the transition between physical and astral without the loss of memory "conscious immortality." People who have achieved the ability to transition and be totally conscious of everything are consciously immortal. They can keep their body alive or let it die and incarnate into a new one with all of their memories. When they dream at night, they know they're dreaming and they can even find information and watch events that transpire in the real world while they're asleep. So we could argue that some of you are already immortal but (and here's the funny part) you still believe that your body will automatically die within about a century.
The Lemurians and Atlanteans had derived immortality in a few different ways. The most notable was their tantric sex meditations. Essentially, they would astral project and conceive a child by non-physical means. The process would cause both parents and the child to become immortal.
During those eras, people were very energetically inclined. Everyone had active chakras and could see at least two planes of existence simultaneously using the third eye. Today many of us are psychic and can see ghosts and auras with enough concentration but ancient humans were many times more sensitive than that. Because they were all sensitive to energy and had active chakras, they were able to passively pull loose energy into their bodies and could live much longer than just a century at a time before reincarnating. They didn't need to eat anything physically. Mind you, these were normal people and not immortals.
For anyone who is sensitive, we know that there are etheric, physical, astral, emotional, and mental bodies. Everyone has one of each and they basically all sit in the same space and overlap. Well there's also a light body which is more geometrical in nature. Unlike the others, not everyone has a fully functional light body. This is why everyone dreams and is capable of astral projection. Everyone has an astral body so it's not a problem. The light body is more intricate and often times there's a procedure that activates it. My apologies, but I can't say I know any one procedure that will definitely activate the light body. Everyone is different so I'd actually be arrogant if I did. What I do know is that knowledgeable people may or may not find you while you're dreaming or projecting in the astral plane and simply teach you how to activate it. Once the light body is fully active, it's possible to translate into any dimension and experience the universe in any way, not just physical and astral.
I suppose all of you scientific people are wondering about genes. Well my understanding is that genes are the physical blueprint of the soul and as such they can be altered by the soul. All of your conscious exploits add an adaptation or a skill to your genetic material. There have even been cases of souls reincarnating with the same face and features almost every lifetime. Conscious immortality makes things rather interesting. This would be no different. Experiencing the transition to an immortal state would naturally change the genes to reflect that. And, really, the immortal state is just the ability to utilize all chakras and the whole of your energy system to nourish every cell in your body for as long as you deem necessary. So what would that gene even look like? If people need energy more than they need physical food then maybe it's possible to genetically engineer an organism which is naturally tuned to the astral plane and whose chakras pull in energy without the intention of the brain. I'm not too sure myself and at this point I wouldn't waste my time with a slow physical procedure like genetic engineering when there are faster ones at the astral level.
I believe that if you want something and deserve it, then you'll get it. The universe is a very well orchestrated system and things tend to work themselves out in time. And until you come to know the exact facets of physical and/or conscious immortality, at least know that you're part of the universe and you have access to as much as you need. Always.