root@OpenWrt:~# ls -al /mnt
drwxr-xr-x    1 root     root           224 Sep 18 06:58 .
drwxr-xr-x    1 root     root           544 Sep  7 16:22 ..
drwxrwxrwx    6 root     root          4096 Sep 18 08:39 usb

background=yes
listen=yes
local_umask=022
check_shell=no
session_support=no
chroot_local_user=yes
user_config_dir=/etc/vsftpd_users
anonymous_enable=yes
local_enable=yes

root@OpenWrt:~# mount
/dev/root on /rom type squashfs (ro,relatime)
proc on /proc type proc (rw,nosuid,nodev,noexec,noatime)
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,noatime)
tmpfs on /tmp type tmpfs (rw,nosuid,nodev,noatime)
/dev/ubi0_1 on /overlay type ubifs (rw,noatime,ubi=0,vol=1)
overlayfs:/overlay on / type overlay (rw,noatime,lowerdir=/,upperdir=/overlay/upper,workdir=/overlay/work)
ubi1:syscfg on /tmp/syscfg type ubifs (rw,relatime,ubi=1,vol=0)
tmpfs on /dev type tmpfs (rw,nosuid,relatime,size=512k,mode=755)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,mode=600,ptmxmode=000)
debugfs on /sys/kernel/debug type debugfs (rw,noatime)
/dev/sda1 on /mnt/usb type ext4 (rw,relatime,data=ordered)
root@OpenWrt:~#

root@OpenWrt:~# cat /etc/passwd
root:x:0:0:root:/root:/bin/ash
daemon:*:1:1:daemon:/var:/bin/false
ftp:x:55:55:ftp:/mnt/usd:/bin/false
network:*:101:101:network:/var:/bin/false
nobody:*:65534:65534:nobody:/var:/bin/false
dnsmasq:x:453:453:dnsmasq:/var/run/dnsmasq:/bin/false

pawel:x:1000:65534:pawel:/mnt/usb:/bin/false
pawel:x:1000:65534:pawel:/mnt/usb:/bin/false
root@OpenWrt:~#

root@OpenWrt:~# cat /etc/vsftpd_users/ftp
dirlist_enable=yes
download_enable=yes
write_enable=yes
anon_upload_enable=yes
anon_other_write_enable=yes
anon_mkdir_write_enable=yes
anon_umask=0000root@OpenWrt:~#