This is a 4.3BSD based version from vacation(1) from NetBSD with OpenBSD additions. Compared to the sendmail version of vacation(8) it supports RFC 3834 "Auto-Submitted:" and RFC 2076 "Precedence:" mail header handling.
To compile vacation(1) on FreeBSD you can simply clone the repository and
type make. There is also a package available via the ports tree. Just
use pkg install bsd-vacation to get the latest binary.
To compile vacation(1) on macOS there is a specific Makefile in the GNU make
format. Just type make -f Makefile.macOS followed by a
make -f Makefile.macOS install. The binary and the man page are can be found
later on in /usr/local/ path.