If I understand correctly, you need to add a new user to your ftp, and have it's homedir /home/vhost/test/httpdoc/somefolder/. I'm not sure if I'm doing this correctly, but I'll try. You need to:
Change this "DefaultRoot ~ psacln" to "DefaultRoot ~".
Then run "useradd YOURUSERNAME". That will add a user with the name YOURUSERNAME to the system. Set the password for the user by "passwd YOURUSERNAME". Then do "ln -sf /home/vhost/test/httpdoc/somefolder/ /home/YOURUSERNAME". That will link the /home/vhost/test/httpdoc/somefolder/ directory to your new user's homedir. Add the user to sshd-denied users (for security). Then you need to add this to your proftpd.conf:
That will deny the user all write access, but allow him to download files and resume downloads.
Please note: I think
this should work. Check the permissions for the /home/vhost/test/httpdoc/somefolder/ directory. I hope this helps you!