I'm not sure where VENDOR gets set - maybe just in the default tcsh binary, but when I su to root (bash) it saves the environment variable, and the ltsp-build-client script looks to see if the VENDOR variable is set, and if it is, uses that value, which in my case is set to "unknown" which then results in the output of ltsp-build-client to be:
/usr/share/ltsp/plugins/ltsp-build-client/common/010-etc-hosts: line 3: /opt/ltsp/amd64/etc/hosts: No such file or directory
error: LTSP client installation ended abnormally
Google didn't help, so hence the reason for this post.
Simply running "unsetenv VENDOR" in my tcsh environment fixed the problem. And as a side bonus, all of the options in the --extra-help are now visible. I wondered why the options didn't match what I was reading in documentation online.