Generated vagrant images does not contain the sudo package. It does contain a /usr/local/etc/sudoers file, but no sudo so there is no way to get root unless reboot in singleuser mode
brd, is this something that you are working on?
after more investigation and discussion with brd at https://github.com/brd/packer-freebsd/issues/17 , it looks like the problem comes with firstboot_pkgs_list which does not work behind proxy. I keep my last suggestion of installing the firstboot_pkg at build time. This will greatly help imho fully automated vagrant workflow behind proxy. I am not fully aware of the details and I would understand it is not practical.
The take away is we don't really want to do this because then the packages installed will be very old as the RELEASE images age. For users affected by this, I recommend they use the packer recipe and modify it to set the http_proxy env variable as part of the vagrant build.