The term MVG is used both online and in brick-and-mortar casinos to describe a person who is a casino’s most loyal customer and, in some cases, it can refer to someone who is crème de la crème of the VIP club.
The abbreviation MVG stands for the most valuable guest, and is something between the most valuable player (MVP) and very important person (VIP).
Granted, the exact meaning of the term will often depend on the one using it, and it may vary from one casino to another or from one player to another, but the gist of it will always remain the same.