(character) Conversion error (in vi) ?

[Ronald F. Guilmette]

Ever since my last system upgrade, some months ago now, vi has been
behaving badly/differently.  Sometimes it gets a bee in its bonnet
over something that, I guess, it is trying to interpret as a (malformed?)
Unicode or utf-8 character or something, and in such cases it refuses
to allow me to edit the file in question.

How can I return it to the old/prior behavior, under which it could
tolerate any old sequence of eight bit bytes without having a hissy
fit?

There must be some option to tell it "don't do that", yes?  (For the
life of me, I don't know why the old/prior behavior wasn't maintained
as the default behavior.  But I wonder the same thing about a *lot*
of different software "upgrades".)