Hi,
In vCloud,virtual machine will be always part of a vApp. Are you going to Move or Copy the VM to vCloud? Firstly,we need to remember that VM's won't be in network on-fly (by default L2(IP/MAC) is not retained).So we will have downtime for sure.
If it is feasible you can try A)Cloning the VM's from VC and importing Cloned VM'S to VCD b) Creating a Storage level Snapshot for those lun's were in VM's are residing,present it to one of the VCD managed Host and register the VM's with a different name and import it to VCD 
Cheers,
Sree | VCIX-5X| VCAP-5X| VExpert 6x|Cisco Certified Specialist
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