Yes we all know the contradictions in the game about Higher Vampires and who is and isn't a Higher Vampire.
However, the reality is that we were all looking at this in the wrong way. "Higher Vampires" are not exactly just a subspecies of vampires, but they are actually a rank.
Think of Higher Vampires as Nobility, when someone is born in a noble family they are instantly considered nobles. However "commoners" can also be granted noble status.
So yes the Higher Vampire is also a status, only some vampires like Regis are born Higher Vampires while others like Hubert Rejk grow into the rank.
Obviously those that reach the rank are still weaker than those that are born and can be killed.
What makes one a Higher Vampire?
Yes thats the big question. At what point does a Bruxa or Katakan become a Higher Vampire? The answer is quite simple - when they gain true intelligence.
Notice that everyone who was called a Higher Vampire in the games shared one common trait which was following their reason rather than instincts. They all have the ability to blend in mong humans for better or for worst. They also don't have to drink blood anymore.
Hubert Rejk for example was insane but he was very cunning as he developed a good plan to fool Geralt, and he was a medic after all, so he could do much more than just use common speech. Notice that he also did not leave any evidence of the killer being a vampire, he obviously could drink their blood but he knew that would reveal the true nature of the killer.
But why is intelligence so important? Well think about it, an intelligent vampire can use their powers in more creative ways as they can strategize and they have the ability to blend in among humans thus making them much more dangerous. However, those that follow their instincts are predictable and are far more easy to defeat.
But, I bet you all want some better evidence so next is...
Orianna
Notice that while Orianna is a Bruxa she is not like the other Bruxa that you fight during The Night of Long Fangs. This is something most never noticed before but note that Orianna DID NOT join the other vampires in attacking Beauclair. She had no intention of following Dettlaff's orders, being the only vampire apart from Regis to do so, which indicates that she is in a different league than other Bruxa. And that's because she can think by herself, while those other Bruxa were more like animals thus they simply followed their "pack leader" i.e. Dettlaff without question.
The Unseen Elder
The Unseen Elder is also not like other Higher Vampires as he is in a league of his own, as even they have to listen his orders. He is proof that vampires grow more powerful over time. So it stands to reason that other subspecies can do the same and so in time certain individuals could reach the rank of Higher Vampires, although they would not be as powerful as those that are born Higher Vampires, but still much more powerful than their primitive brethren.
Links
There are also Vampires that I call "links" which are not Higher Vampires yet but possess some intelligence and are usually under the care of real Higher Vampires like the "Sisters of the Night" who obey the Queen of the Night or the "The Bruxa of Corvo Bianco" who was under Dettlaff's wing. Notice that in the latters case her entry clearly states that she was more "intelligent" than other Bruxa, yet she was clearly not as intelligent as Orianna.
Lower Vampires
This also explains on what makes other subspecies considered "lower" when they also have different power levels i.e. Garkains are stronger than Fleders. However, they are all considered lower because they can never reach true intelligence and they are always bestial in appearence. Thus they could never "blend in" among humans.
The real ranks
So the real ranks among vampires starting with the weakest would be more or less like this:
Low Grade - always bestial in nature and appearence
Fleder, Ekimma, Garkain
High Grade - follows instincs rather than reason
Average Bruxa, Nosferat, Katakan, Alp, Mula
Links - more powerful than high grade equivalent, possesses limited intelligence and some ability to blend in among humans
The Bruxa of Corvo Bianco, Gael, Sisters of the Night
Higher Vampire (rank) - much more powerful than high grade equivalent, possesses true intelligence and can perfectly blend in among humans
Orianna, Hubert Rejk, Queen of the Night
Higher Vampire (born) - Vampires born with enormous power and intelligence, which are also impossible to truly kill except by another vampire
Regis, Dettlaff
Elder - ancient Higher Vampires who gained extraordinary power, even Higher Vampires must obey them
The Unseen Elder