Thank you for your PR (and so sorry for the very very long turnaround time...)

What assumptions are being made here, and how robust is this with respect to different possible hosts (eg, different operating systems)?

Yes, I see, that's an important question.
I have updated vterm--tramp-get-shell. Now it should be pretty robust.

    (if (or (eq first 'login-shell)
            (and (consp first)(eq (cadr first) 'login-shell)))
        (let* ((shell (ignore-errors
                        (with-output-to-string
                          (with-current-buffer standard-output
                            ;; Use a shell command here to get $LOGNAME.
                            ;; Using the Tramp user does not always work
                            ;; as it can be nil, e.g., with ssh host configs
                            (unless (= 0 (process-file-shell-command
                                          "getent passwd $LOGNAME"
                                          nil (current-buffer) nil))
                              (error "Unexpected return value"))
                            ;; If we have more than one line,
                            ;; the output is not the expected single passwd entry.
                            ;; Most likely, $LOGNAME is not set.
                            (when (> (count-lines (point-min) (point-max)) 1)
                              (error "Unexpected output"))))))
               (shell (when shell
                        (nth 6 (split-string shell ":" nil "[ \t\n\r]+")))))
          (or shell second))
      first)

• $LOGNAME should be defined in every POSIX-compliant system,
  see https://pubs.opengroup.org/onlinepubs/007908775/xbd/envvar.html
  If it is not defined, vterm--tramp-get-shell will return the fallback value or the default shell ("Unexpected output" -> shell is nil).
• getent passwd returns the passwd entry for the specified user, independently from the used name service (I also tested it with LDAP),
  see https://en.wikipedia.org/wiki/Getent
  The getent command is widely available in Unix systems [1].
  However, MacOS X does not provide it. Thus for MacOS X hosts, vterm--tramp-get-shell will return the fallback or the default value ("Unexpected return value" -> shell is nil).

[1] from the FreeBSD manpage for getent: "A getent command appeared in NetBSD 3.0, and was imported into FreeBSD 7.0. It was based on the command of the same name in Solaris and Linux."
https://man.freebsd.org/cgi/man.cgi?query=getent

So, it works for most Unix-like remote hosts, including GNU/Linux and the BSDs.
For MacOS X, the probing fails (since getent is missing), but the function should return the fallback value (not tested, since I have no Mac).
I don't know about Windows.. I guess with the Linux subsystem it should work.

If you deem it too unstable, I would argue for changing the default value of vterm-tramp-shells back to the original value but include the 'login-shell functionality as an optional feature.

Thank you.

I think we can go ahead with this. Here are some comments to make the solution more robust (and cleanup a few things):

• Tramp supports all sorts of protocols and systems. While I expect ssh to be the most common (by far), we should not make too many assumptions about it. I suggest we turn the login-shell method you introduce to be the default for ssh, but let the user pick it for other protocols if they want to.
• In this vain, I think it would be nice to check that we have a corresponding entry in vterm-tramp-shells for the method being used and error out when we don't
• Restrictions and assumptions should be documented in the code (as you did in the previous message). More explanation is welcomed too (e.g., why (nth 6)? You could paste an example so that readers would konw what to expect)
• Please, format your code and check that docstrings are complete and accurate, including defining all the arguments function take
• Please, add an entry to the README.

Thank you again!

I have updated the PR.
Now, there is extensive in-code documentation and a extended docstrings. I also added a README entry with examples.

I suggest we turn the login-shell method you introduce to be the default for ssh

I set it to default for ssh and scp, since scp are ssh servers in most cases. What do you think?

In this vain, I think it would be nice to check that we have a corresponding entry in vterm-tramp-shells for the method being used and error out when we don't

I am not exactly sure what you mean by that.