Just tested to install LUCI for vsftpd over OpenWRT 21.0.2.
/usr/lib/lua/luci/controller/vsftpd.lua:21: attempt to call field 'loadc' (a nil value)
/usr/lib/lua/luci/controller/vsftpd.lua:21: in function 'v'
/usr/lib/lua/luci/dispatcher.lua:1219: in function 'createtree'
/usr/lib/lua/luci/dispatcher.lua:693: in function 'menu_json'
/usr/lib/lua/luci/dispatcher.lua:794: in function 'dispatch'
/usr/lib/lua/luci/dispatcher.lua:479: in function </usr/lib/lua/luci/dispatcher.lua:478>
Upper message showed even refresh management page.....
How can roll back or remove that LUCI to return the normal page ?
I do not think that there is any official LuCI support for vsftpd. Did you install a third-party app from somewhere?
One easy way with most routers is to use reset and revert all packages & settings to the plain installed firmware image.
Sure, you need to reinstall the possible add-on packages that you have installed, but that would be a way to remove all possible installed unknown extra stuff. You probably have a valid settings backup, so you could restore that (plus install the add-on packages)