I set up vacation according to the README files and man pages.
I added a link from /usr/bin/vacation to /etc/smrsh, and ran
vacation to set up the message and .forward files.  Also ran
vacation -I to initialize the database.

Result is that incoming messages are replied to with a completely
empty message (just headers).  The symptoms are that vacation closes
the pipe to sendmail and exits before sendmail has a chance to finish,
sendmail gets a broken pipe, and vacation gets a seg fault closing
a FILE pointer that was already closed.

The quick solution is to change vfork to fork.
The better solution would be to use popen("...sendmail", "w")
and pclose(), rather than pipe/fork/rearrange fds/fdopen/close.

Note: there is a newer source RPM in contrib/libc6 which has the same
problem with vfork, but has added code to ignore case when comparing
headers.