Having gotten the official (per https://forums.freebsd.org/threads/52717/ ) Vagrant image up, including steps in bug 201904, I'm hitting a problem: sudo segfaults immediately, so I'm unable to become root and do anything useful.
freebsd/FreeBSD-11.1-STABLE (virtualbox, 2017.09.06)
I've confirmed that this is specific to the RelEng-maintained Vagrant image; the Chef-org-maintained "bento" image of 11.1-RELEASE-p1 is fine.
This is a bug in sudo, see the linked bug.
*** This bug has been marked as a duplicate of bug 222510 ***