I have a similar setup at home.
Create the user, once it's created edit /etc/passwd as root.
Look for the line where the user is listed.
The last part is going to have the home directory and then the shell.
Change the home directory to /srv and the shell to /bin/false.
Then the user cannot login to the shell.
Then you edit /etc/vsftpd.conf
That's from memory, but that should work.