I actually went through this recently with a customer.
You can upgrade vCD with nothing else changed. It'll detect vCenter version and determine next steps. If vCNS isn't upgraded, then you can do that after the vCD upgrade is complete.
If you happen to be in the scenario where vCenter is at 5.0, you can upgrade vCloud first. It'll show as unsupported when vCD 5.5 comes online. Then upgrade vCNS to 5.5, then vCenter to 5.1 or 5.5 ... your choice. Some people just wanted to jump directly from 5.0 to 5.5 with vCenter, and that's actually possible.
I don't think we do a really hard compliance check with vCNS like we do with vCenter. More that vCenter is supported and there is a vShield/vCNS attached to it.
So long as you get yourself into a supported configuration before using things heavily, then everything should be fine.
Again ... just note I'm not in the office (right now or for a week or so), so I may not reply very quickly.