Just wanted to inform you, that we tested it with "forced elastic mode". It did work like a charm - with both new and existing vDCs. With existing vDCs the RP configuration was changed to "expandable" as soon as a vm was powered on, or RAM allocation was changed within vCloud Director. We are now able to use 100% of allocated RAM. It is not possible to use more RAM than allocated (eg. powering on a new vm, when no RAM is available)
So - all is well. Thank you.