To get the value of character under cursor: ga

(where ^V means "hit Ctrl-V, except if you use Ctrl-V to paste, in which case you should hit Ctrl-Q instead):

    By decimal value: ^Vnnn (with 000 <= nnn <= 255)
    By octal value: ^VOnnn or ^Vonnn (with 000 <= nnn <= 377)
    By hex value: ^VXnn or ^Vxnn (with 00 <= nn <= FF)
    By hex value for BMP Unicode codepoints: ^Vunnnn (with 0000 <= nnnn <= FFFF)
    By hex value for any Unicode codepoint: ^VUnnnnnnnn (with 00000000 <= nnnnnnnn <= 7FFFFFFF) 

Notes:

- This works on the command line (e.g. when entering a pattern/regular
  expression) too.

- In all cases, initial zeros may be omitted if the next character typed is not
  a digit in the given base (except, of course, that the value zero must be
  entered as at least one zero).

- Hex digits A-F, when used, can be typed in upper or lower case, or even in
  any mixture of them. 

To search & replace:

Check the help for \%u:
                                /\%d /\%x /\%o /\%u /\%U E678

\%d123  Matches the character specified with a decimal number.  Must be
        followed by a non-digit.
\%o40   Matches the character specified with an octal number up to 0377.
        Numbers below 040 must be followed by a non-octal digit or a non-digit.
\%x2a   Matches the character specified with up to two hexadecimal characters.
\%u20AC Matches the character specified with up to four hexadecimal
        characters.
\%U1234abcd     Matches the character specified with up to eight hexadecimal
        characters.