Simple Explanation:

In Old World of Darkness we have variety of supernatural-related beings hiding from humans, often living in secret from each other. Mages are capable of bending reality to their will. Vampires are selx-explanatory. Hunters are those who hunt the supernatural to protect humanity.

Mages have been in a brutal war since XIII Century, between those who use magic and Technocracy, who wants to replace magic with science (which is just magic operating on different rules). Technocracy is winning and has run Pogroms (actual I-kid-you-not in-universe name) of “Reality Deviants” - Mages, Fae, Werewolves, Wraiths etc. Except in London, where both sides are relatively chill and local Technocracy believes polite debate is far more effective than gunning down Mages in the street, like barbarians. Most drastic events of the Mage metaplot had minimal effect on Mage society in London.

In 2012 Coallition of Hunters called Second Inquisitions condone a takeover operation of London, slaughtering most of Vampires in the city. London’s Prince, ruler of Vampires in a given city, effectively rules from exile, afraid to show up in London unless absolutely necessary. Hunters use, ever-present in London, cameras to pick Vampire activities, track them down and kill them like dogs. London is refered to, by Vampire players, as “vampire ground zero” or “final death trap”. Playing Vampire in London is described as playing a postapocalyptic game surrounded by mortals unaware the nukes hit.

These two exist in the same city at the same time.