I have many households of vampires but they are households that I play at some point. However, when I am not playing them, they seem to manage their energy just fine. But, they are also all fully trained in vampire lore and most of them have at least 100 plasma packs in their inventory. I have not turned an NPC into a vampire and just left them on their own like you did. So I find this very interesting. With that said my first thought would be to have your sim mentor them in fishing since fish can be converted to plasma packs. Since we don't know if an NPC will use a computer to purchase them or if they have enough money to continually do so, teaching them to fish might help. An NPC also might not have a house with an appropriate place to sleep to regenerate their energy. Many of the NPC sims don't actually live in the world so I think it would be hard for them to regenerate properly. With new vampires being subject to sunlight damage, that could be a problem. I would think that things like this would be accounted for but I truly do not think they are. When I have ran businesses with my sims, the employees, which are NPCs, always show up tired and thinking about beds. The NPC also probably won't improve their skills or vampire powers on their own if other NPC sims I have seen are an example. I suspect that the reason their energy stays so low is because they are going outside in the sunlight a lot and it keeps draining their energy to keep them alive.
So with all of that said, if I were you, I would find out where the NPC lives and make sure they have an appropriate place to sleep with a decent bed so that they can regenerate energy. Teach them to fish and take them to a library to have them study up on vampire lore. From the fishing create as many plasma packs as you can. Then I would save, go back to manage worlds and click on their household to change it back to an unplayed household and hopefully doing that much for them will make them better able to survive. It could just be that the particular sim you chose to make a vampire is glitchy or something and this is just an unusual circumstance.
I hope you will post an update to let us know what you do and what ends up happening. Good luck!
