Regan MacNeil has been theorized to exist in GTA Vice City, mainly due to the existence of a related entity called Pazuzu. In the famed exorcism movie, The Exorcist, Regan MacNeil gets possessed by an ancient evil known as Pazuzu. Since Pazuzu is rumored to exist in Vice City, it might be possible that it may have attacked civilians.
According to an unknown source, Regan MacNeil could be seen levitating in the alleys behind the Haunted Basketball Courts. However, this report is entirely false, but Regan MacNeil and The Exorcist have been referenced elsewhere. Rockstar published an article relating to GTA Vice City. In the article, To Live and Die in L.A. (1985) has been honored, which was directed by William Friedkin, the director of The Exorcist. Moreover, The Exorcist reference was explicitly mentioned by Rockstar in the article itself. However, the article is only for information purposes and doesn't prove the existence of Regan in GTA Vice City.