Get the solution from Vim Wikia post here, already tried this by myself and worked like a charm :)

If you ever get this error while trying to add cron entries:

crontab: temp file must be edited in place

Then the solution is very simple. You just need to create ~/.vim/ftplugin/crontab.vim and add this one line:

set backupcopy=yes

Save the file and you can try to add cron entry again.