Mainly just to add a different viewpoint that takes a different perspective regarding some things in the long comment.
Outside of it being your world and subject to your whims. It is not this Earth and therefore is not subject to the cultural specifics of it. It may be close, but small adjustments can make a huge difference.
Complete sentences versus incomplete and slang versus not slang:
I know several people who speak multiple languages and they use slang for each. One friend speaks seven and you would never know where he was from because he hides his accent in order to fit in with his colleagues. Caring about knowing slang is dependent on the specific character and other cultural things mentioned later. Bottom line, if you spend a lot of time around people that use slang, you will use slang.
Incomplete sentences versus complete sentences is totally irrelevant as a set in stone argument. If the vampires are seen as high society who expect a certain level of decorum then we could see complete sentences in that setting. On the other hand, if werewolves tend to be young then we might see more slang and adjusted speech with them.
The main character "Atlas" may prefer modern speech and keeping up with younger people when he is not in a formal setting. This would drive a him to be more like a college age student/ young adult than someone "high class". This seems a bit accurate due to where we see the character early on.
side note: With him being originally from the time of the Ancient Greek (and we don't know where he went from there,) being slapped with a ruler on the hand is more invalid if Atlas is speaking relating to his own experience. When you consider that the vampire who turns him would be older, the discipline used could be even more foreign. Don't know the specific scene, but relative to Atlas, the idea of being slapped on the wrist with a ruler is much younger than he is.
Last note: While old money and royal courts have an internal cultural complexity, that complexity comes from internal politics and external politics of nearby regions. Things can change quickly depending on a number of factors. That is why things today have changed compared to 300 years ago, which have changed compared to 600 years ago. High society strived to be fat due to the association with being rich at one time, but not anymore.
A lot of how the vampire society works would be based off of its ruler. A ruler who places importance on blending in with the general public would actually cause vampires in general to seem like just any average modern younger person. When I say younger, I mean not quite over-the-hill by the way. On top of that, I would also argue that if you lived for hundreds or thousands of years and kept your youth, chasing adventure and life in the moment would be more important than what most older people place importance on. I buy driving society from the background and living in the moment much easier than trying to act like human royalty. Especially since they rule through power. If you can destroy your enemies easily, then how you act is less based off of acting a certain way for your subjects approval than doing what you want.
That can be dived into a lot deeper regarding cultural and political impacts, but this is already longer than necessary.
All of that is really to say that you should take the story in the direction you want, and that there is no requirement for it to be written a specific way to seem realistic to a specific person. Everyone will have an opinion on what makes more sense to them, but you know your world better than anyone.
I have enjoyed the story so far. I have noticed the improvements over time. I look forward to the next piece of the story!