The sudoers file should always be edited with 'sudo visudo' (see 'man
visudo'). 'visudo' runs $EDITOR, which is not necessarily 'vi'. You can
define the EDITOR variable (here I assume 'nano' is your favorite text
editor) when running the command:
$ sudo EDITOR=nano visudo
But you probably want to define EDITOR once for all in the personal
configuration file of your shell (here ~/.bashrc, the personal configuration
of Trisquel's default shell: Bash):
$ echo EDITOR=nano >> ~/.bashrc
Be careful with what you type: there must not be any space around "=" and you
need two ">" to append a line to ~/.bashrc (one single ">" would substitute
its entire content).
