In the Unix world I think there were versions of full-screen editors
well before "vi" was fully fledged. I used one called "fred -- the
friendly editor", initially IIRC on a PDP 11/60 running 7th Edition Unix
at the University of Calgary, though at the time it was contemporary
with "vi" (1980). It was, I believe, basically also an adaptation of
Unix "ed". It had a modeless "open mode" controlled by the cursor keys
allow one to move about and change text at arbitrary locations on the
screen, though it could also be used in a more line-oriented way like
you would expect a full-screen "ed" to work -- in this mode it still
maintained a full screen's worth of context and one could "page" up and
down in the document being edited but typically one inserted/appended
lines in a modal manner, or moved or deleted or ran other commands on a
line or block of lines at a time.
fred was original released for v6. FWIW: I think it came out of Cornell. I know we had it at CMU in 1977/78 and ran it on Perkin Elmer Fox terminals mostly, since that is what most of the UNIX boxes we had. I'm pretty sure that I have the sources somewhere. There was nothing like termcap; it was hard-coded to the terminal being used in some manner, probably reading something static like /etc/ttys and knowing the site's set up. But as you said, it was small enough to run on 11/40 class UNIX systems which helpful since 40s and 34s were what we had the most. We did not have vi nor emacs in UNIX-land which was still PDP-11's (EMACS was on the PDP-10s).