De man less
,
v Invokes an editor to edit the current file being viewed. The editor is taken from the environment variable VISUAL if defined, or EDITOR if VISUAL is not defined, or defaults to "vi" if nei‐ ther VISUAL nor EDITOR is defined. See also the discussion of LESSEDIT under the section on PROMPTS below.
Basta definir a variável de ambiente padrão EDITOR
de acordo com seus desejos, por exemplo, export EDITOR=vim
em ~/.bashrc
ou algo parecido.