the ESXi host is not powered down ... the host is put into maintenance mode.
In an ideal world, you would have vCenter/vShield in a different cluster than your vCloud deployed VMs.
For this to happen, you need to ensure DRS is at full auto mode, and that vMotion does actually work.
Aside from that, if you create a provider vDC and prepare ALL hosts, they may all try to enter maintenance mode on their own.
This is a bit of an awkward scenario ... since you can't move VMs onto a host entering maintenance mode.
so even though if failed, if one prepared fine ... see if you can prepare the failed host ... this time it may work.