Yup, that's definitely it. Deleting the subvolumes under /var/lib/lxc/vagrant-local-machine-*/rootfs/srv/disk/{current,snap_} freed up /var/lib/lxc/vagrant-local-machine-* so that lxc-destroy worked.
Yup, that's definitely it. Deleting the subvolumes under /var/lib/ lxc/vagrant- local-machine- */rootfs/ srv/disk/ {current, snap_} freed up /var/lib/ lxc/vagrant- local-machine- * so that lxc-destroy worked.