Edit crontab with VIM

How to change crontab editor from NANO to VIM ?

Usually google searches give following method, But it didn't work for me,

export EDITOR=/usr/bin/vim

crontab -e

or

EDITOR=vim crontab -e

At same time, following is working fine for me.

export VISUAL=/usr/bin/vim

crontab -e

VISUAL=vim crontab -e