Hi Gabrie,
If you use "service vmware-vcd stop" (or "service-vmware-vcd restart") in order to stop (restart) the vCD you will stop the vCD process without paying attention if there are any running tasks or not. All running tasks will be cancelled/failed.
If you use the "cell-management-tool -q true" in order to quiesce (Prevent creating new tasks) and then with the shut down option it will be the better way to do it.
In this case vCD will check if all running jobs are finished or cancelled by the user and then it will initiate the shut down process.
Basically with the cell-management-tool vCD will wait for all running tasks to become finished before the actual vCD shut down.
This quote is from vCD 5.1 install guide (http://pubs.vmware.com/vcd-51/topic/com.vmware.ICbase/PDF/vcd_51_install.pdf):
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Use the Cell Management Tool to Quiesce and Shut Down a Server
Before you upgrade a vCloud Director server, use the cell management tool to quiesce and shut down
vCloud Director services on the server's cell.
vCloud Director creates a task object to track and manage each asynchronous operation that a user requests.
Information about all running and recently completed tasks is stored in the vCloud Director database. Because
a database upgrade invalidates this task information, you must be sure that no tasks are running when you
begin the upgrade process.
With the cell management tool, you can suspend the task scheduler so that new tasks cannot be started, then
check the status of all active tasks. You can wait for running tasks to finish or log in to vCloud Director as a
system administrator and cancel them.
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Ivaylo