Yes, Virtual IO DR works for both adding and removing virtual disks. You also seem to have done the steps necessary to make sure the vdisk can be removed (making sure it is unmounted and not in use).
I'd suggest checking /var/adm/messsages on the guest and see if any of the kernel drivers reported an issue or something holding the disk open for some reason. Also, see if there are an message in the ldmd SMF log (/var/svc/log/ldoms-ldmd:default.log)
(and checking that 'drd' is online in the guest is always a good thing to double-check)