Both 'dpkg-reconfigure grub-pc' then selecting the /dev/vd* disks and manually running 'grub-install /dev/vda' (and then /dev/vdb) do work.
Both 'dpkg-reconfigure grub-pc' then selecting the /dev/vd* disks and manually running 'grub-install /dev/vda' (and then /dev/vdb) do work.