If your 'encoding' setting is utf-8 (usually set somewhere early in
your vimrc if your system doesn't default to it) you can type any
character in Vim if you know its codepoint number, see ":help
i_CTRL-V_digit". You should NOT change 'encoding' after Vim has
finished starting up; for some other values of 'encoding' there exist
characters which Vim cannot represent in memory.
For instance, to enter the codepoint U+2019 when in Insert or
Command-line mode, type Ctrl-V followed by u followed by 2019
To replace all right single quotation marks by apostrophes in the
current editfile, you would then type
:%s/’/'/g
where the "replace from" character ’ is obtained by Ctrl-V followed by
u2019 and the "replace by" character ' is the apostrophe.
Best regards,
Tony.