I hacked it up to work on 5.2.1-RELEASE through -CURRENT.. might be broken on 4.x (haven't tested it).
# fetch http://bsd-unix.org/downloads/vnstat-1.4.tar.gz
# tar xvfz vnstat-1.4.tar.gz
# cd vnstat-1.4
# ./freebsd-install single (single user ~/.vnstat ~/bin)
or..
# ./freebsd-install...