nano /etc/vsftpd.conf
# modify these lines
write_enable=YES
chroot_local_user=YES
# add these lines
userlist_enable=YES
userlist_file=/etc/vsftpd.userlist
add the user to /etc/vsftpd.userlist
usermod --home /var/www/html/ username
chown -R username /var/www/html
chmod -R 755 /var/www/html
7 is user, 5 is group, 5 is other
7 is binary 111, 5 is binary 101
111 = read (yes), write (yes), execute (yes)
101 = read (yes), write (no), execute (yes)
So, the user can read, write, execute but other people can't write.