I’m not familiar with vi so rather than struggle with it via crontab -e I used Cronnix to edit my tab. Simple enough.
my cron tasks weren’t running on leopard and I found a better, more flexible, solution: http://www.tuppis.com/lingon/
“Lingon is a graphical user interface for creating an editing launchd configuration files for Mac OS X Leopard 10.5. You can use launchd on a Mac to launch scripts and applications whenever something special happens or at a specific time or periodically.”