Still what use are the
permissions on a file if it is controlled from the directory?
The permissions on the file control access to the contents of the file.
Not being a vsftp user myself, I took a quick look at the man page and it looks like there are some commands for anonymous users that permit uploads with out allowing deletes. There is also a separate option that restricts virtual users to the permissions of a anonymous user, not sure if it will work, but have you tried playing with these options?