A vCentre runs one or more datacentres but only for one organisation or org umbrella. A PDM can connect to and control multiple "trusting" parties.

I (we) have several customers with PVE deployments and VPNs etc to manage them. PDM allows me to use a single pane of glass to manage the lot, with no loss of security. My PDM does need to be properly secured and I need to ensure that each customer is properly separated from each other (minimal IPSEC P2s and also firewall ingress and egress rules at all ends for good measure).

I should also point out that a vCentre is a Linux box with two Tomcat deployments and 15 virty discs. One TC is the management and monitoring system for the actual vCentre effort. Each one is a monster. Then you slap on all the other bits and pieces - their SDN efforts have probably improved since I laughed at them 10+ years ago. VMware encourage you to run a separate management cluster which is a bit crap for any org sub say 5000 users.

PDM is just a controller of controllers and that's all you need. Small, fast and a bit lovely.