I had the same problem at one point. Once the user is successfully logged in the connection is established, when vsftp tries to put that user in their home directory and its not there or the permissions are wrong you get a quick disconnect. The ssh default is going to be "/home/username" unless you specified otherwise. So what is prolly going on is that when the "new" users log in they are going to a directory "/home/username" in which case I'm sure the proper permissions airn't set. I think you have to options:
1. Make sure your "new" users home directories are set to the "/ftpdocs" and not "/home/username".
2. Change the permissions on the "/home/username" directories.
Hope this was of some help any more questions just let me know I'll try to help. Also try this site out for some more helpfull tips http://vsftpdrocks.org