Is VMS a UNIX variant? For all practical purposes Linux, BSD and OSX are flavors of UNIX.
No. VMS is its own beast. It was a rock solid OS by Digital Equipment Corp (DEC), back in the day. I never worked a whole lot with VMS, but from what I saw it was excellent in just about every way that mattered.
From wikipedia:
OpenVMS is a multi-user, multiprocessing virtual memory-based operating system (OS) designed for use in time sharing, batch processing, real-time (where process priorities can be set higher than OS kernel jobs), and transaction processing. It offers high system availability through clustering, or the ability to distribute the system over multiple physical machines. This allows the system to be "disaster-tolerant"[9] against disasters that may disable individual data-processing facilities. VMS also includes a process priority system that allows for real-time processes to run unhindered, while user processes get temporary priority "boosts" if necessary.[10][11][12]