vCloud cannot move a vApp between Org vDCs without downtime. There is a lot of metadata that prevents this from happening, as the DB needs to be updated. It would be best to power them off.
If these are within the same provider and vCenter, then once powered off it'll be a rather quick. If the VMs are in a different vCenter, then the move will take care of everything ... but will take longer since there is a potential for needing an export and import process.
SvMotion is not a supported way of migrating between Org vDCs.