An Elastic vDC allows you to add more resources to a Provider vDC when they become consumed/constrained.Dave thanks Timo Sugliani for helping him find this configuration which he has now shared on his blog. The complete step by step instructions are there.
Why would you need to do this? Well first off this only works with the Pay-as-you-go Allocation model. Pay-as-you-go is exactly that, you have no idea as a provider on how many VMs will be deployed. Potentially there is the possibility that all your resources will be consumed, and your consumers will not be able to deploy any more workloads. This is where Elastic vDCs come in, you can add more resources to a PvDC on the fly. So how do we do it? --Dave Hill