VMS wasn't all vax assembly either. VMS and a lot of VAX system software was mostly written in BLISS [1], a high level language intended for systems programming that was created about the same time as C. It was all there for customers to read in a big stack of microfiche cards delivered with each machine. I remember being shown the code for the VMS scheduler or maybe the dispatcher which was 'a page or two of (VAX) assembly' - I recall it was pointed out that it was one of the (few) places in VMS that did use assembly.

I don't know how portable the BLISS code for VMS was, but there were BLISS compilers for several other machines besides the VAX.

1. https://en.wikipedia.org/wiki/BLISS