I am using it for the wordpress that I run.
The folder /var/www/wordpress/wp-content has only root write permission.
No one else has right permission to it.
I want to give the vsftpd ftp also write permission for it.
But no other user should have that permission (except from the root).
I don`t know on Ubuntu but the deamon itself should use user vsftpd( check /etc/passwd) however this won`t give you access to the files. You need to setup new user - wwwpress for example setup his home directory as /var/www/wordpress/
Change the owner of the files in the wordpress install dir
chown -R wwwpress:wwwpress /var/www/wordpress/
And then ftp in with wwwpress. That way user wwwpress will be able to modify the wordpress files and your wordpress instance will remain protected.