General :: File Name Extensions With VIM (VI Improved)?

GNU/Linux kernel 2.6, Slackware. VIM - Vi IMproved 7.1 Huge version without GUI. When I quit an editing sesion vim creates a backup file with the same name as that of the edited file but with a '~' appended. I would like the name for the backup file to be the following: same base name as that of the edited file and edited file extension replaced by 'bak'. Example:

Input file//////////////backup file after editing
----------//////////////-------------------------
foo.txt/////////////////foo.txt~
foo.txt/////////////////foo.bak

The first line corresponds to what vim now does. The second one is what I want vim to do.