The Vistani are more hugely important to Ravenloft for two reasons: 1.) They're the only ones who are aware of the nature of the demiplane and 2.) They have amazing powers of divination and cursing, far beyond what "known magic" can do. They hold a special place in Ravenloft as being basically uber-powered humans who are cunning enough to defy darklords and get away with it. Making them just another FR ethnic group absolutely shatters that mystique.
The Mists are another ubiquitous element: they transport you into, out of, and around Ravenloft. The spring up anywhere, can take you anywhere, and only seasoned travellers (or the Vistani) know how to use them to get to where THEY want to go. Additionally, the mists can help seal the borders in many domains.
Oddly, this is one of the easier parts to port: two of the bigger faiths in RL (The Lawgiver and Morninglord) and exports of FR anyway (Bane and Lathandar, the latter via Jandar Sunstar). A few others are other RW deities (Benelus, Osiris) or unique to a single domain (Zhakata) but the biggest loss would be the Church of Ezra, a Catholic church replacement complete with schisms and denominations rather than a single deity; not sure it would work in Faerun since part of its allure is its widespreadness...
Bzzzt! The Dark Powers were the ones who TRAPPED those creatures. They're the overseers of the domain, they are the ones who are the jailers. They made the pact with Strahd, cursed many a dark lord's ambition, tortured Soth and even imprisoned Vecna for a short while. Whenever a being does evil, it draws a chance the Dark Powers will notice and "reward" them with some boon tied with a curse; enough of such actions and the evil-doer risks transforming into a creature of the night or becoming his own minor domain lord. They also are the reason magic works funny in RL; enhancing necromancy, stopping ALL planar travel (even summons) and making divination unreliable. In short, they are powerful enough to mess with the powers of Gods and a major reason why Ravenloft IS Ravenloft.
Each realm has its own Dark Lord, and many are pretty much classic gothic horror monsters with serial numbers filed off. There are versions of Dracula, Frankenstein, the Wolfman, the Mummy, Jeckyl/Hyde, a Voodoo zombie lord, a Vlad the Impaler, a ghost lord, the Weird Sisters of Macbeth, etc. Each gets its own domain which represents the setting (a vast desert, the mountains of Switzerland, a Caribbean Island, and lots of Eastern Europe). Domains can range from bronze age to renaissance tech as well.
I'd call it an IP theft more than a compromise; you're taking the barest wisp of the idea and slamming it into the Realms. There is an easier way...
Ravenloft, by its nature, is modular. That is, the Mists come in sweep you up, put you in some situation, and at the end of it release you. Its how the "Night of Horror" mode of RL play worked through most of 2e; the idea of RL as "full world" came much later. Go back to the Night or Horror model; sweep up some 1st level adventures, let them wander around a domain for 15 levels, return them when done. You can add all the variant rules needed in the Appendix (slimmed down a bit), no PC info (since their not natives) and in the end, you're not forcing Ravenloft's IP onto Faerun (disrupting both in the process).
(Planescape would be easily done this way as well; substitute "mists" with "portal" and "domain" with "Sigil")
