Use
set fileencodings=utf-8
(with an s at the end) which can contain a list of different encodings. Vim will try the encodings listed, from left to right, until one works and it will set fileencoding to that encoding. If none work then fileencoding is set to an empty string which will result in default behavior.
Also it would probably make sense to add that to your vimrc so you aren’t constantly doing that.