Real vampires can superinfect each other making each other stronger as a vampire.
This is why real vampires forming groups with other real vampires can be very beneficial to all of the vampires in that vampire group. The wikia in this link can be used to help vampires start a vampire group in their area. If you aren't a real vampire this superinfection page does not apply to you, however the how to get turned into a vampire can help you become one.
Superinfection works because it is an endogenous retrovirus, and every time any kind of virus goes to a new host it mutates a little to adapt to the new host; which is how and why if two vampires do the turning process on each other, they will both become a bit stronger. Just like the flu virus and other things like the HIV, it is constantly changing and evolving because of this reason.
As long as when changing people into a vampire, vampires only choose people that have something to contribute as opposed to those that will weaken the race. The vampire endogenous retrovirus will always continue to evolve into something better. However if the vampire makes a bad choice in who they turn, they will make themselves weaker as a vampire. Not to mention will turn people into vampires, that will only result in being an insult to the vampire race.
Due to the turning process being a two way thing, The vampire is also changed a bit in the process as the endogenous retrovirus that is already in them adapts to the other persons DNA as well. This doesn't sound like much of a big deal, until you start thinking in numbers of about 20 or more vampires. Vampires that form groups and superinfect each other is the fastest and best way to accelerate the rate of our own evolution as a vampire race. Superinfection only works with real vampire, it won't work for energy vampires because they aren't real vampires.