Hi,
I believe best practice is to run 2 clusters, one for management and one for vCD hosts.
Create yourself a new cluster using 3 of your 5 ESXi nested hosts and don't have an VMs running on it, and you should be able to prep vCD on that cluster.
The other two ESXi nested hosts can run in a cluster too, and they would run your AD, VC, vCD and vShield VMs.
Cheers,
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