start on vagrant-mounted (upstart) equivalent in systemd
Solution 1:
My understanding of the Vagrant boot sequence is that Vagrant mounts /vagrant (and any other shared filesystems you have defined) using SSH, after the boot sequence has completed.
As such, you'll probably want to run systemctl start my.service in a provisioner, with run: always specified, so the provisioner runs on every boot, not just when the VM is initially created.
This wouldn't be necessary if systemd had the ability to detect when a filesystem was mounted, but to my surprise I can't find any indication that systemd supports such functionality.