well, the basic thing you need is a username to login through FTP on your server
how-to FTP server
and probably you don't want to allow that user to have a shell access, so you would add a user like:
#useradd -s /sbin/false -d /path/to/home-directory/username username
YES, "username" twice (2 times),
this will create a local user on your server with "username" and a "home" directory "/path/to/home-directory/username", but without shell login permissions (-s /sbin/false)
Then you edit the "/etc/passwd" file:
and edit the line with "username::path/to/home-directory" <-- change to /path/to/web/directory (where the web site files are)
then you will need to "chown" that directory and allow others (apache, etc...) to read from it
#chown username.group /path/to/web/directory -vR
#chmod 755 /path/to/web/directory
this gives the "username" ownership to "path/to/web/directory" and you basically let others to read/execute it (and the files inside).
Now you have to set the password for "username"
...enter the password
...repeat the password
Now if you have vsFTPd configured to allow local users to login you will be able to login and upload/change/delete files.
PS. It's a basic FTP configuration, but I see you are using Debian Squeeze which I don't know and I wrote all this for RHEL/Fedora systems
Anyway, good luck