> Unix (...) first source license was sold in 1975.
> VMS was released in 1977 as VAX/VMS on the VAX series of computers. Before VMS the DEC hardware ran various other operating systems such as RTX-11, TOPS-10, and optionally AT&T Unix
Before VMS there was RSX-11 (note RSX not RTX):
https://en.wikipedia.org/wiki/RSX-11
"From 1971[5] to 1976 the RSX-11M project was spearheaded by noted operating system designer Dave Cutler, then at his first project.[5] Principles first tried in RSX-11M appear also in later designs led by Cutler, DEC's VMS and Microsoft's Windows NT.[6][7][8]"
In Dave's words:
https://tech-insider.org/windows/research/1992/11.html
"RSX-11M was introduced in 1973, 18 months after we started building"
Note Dave Cutler and note the years. I'd say the history is comparable.