Both DEM Worker and the proxy agents follow the active-active HA configuration and failover process is automatic.
If the worker service stops or loses its connection to the repository (Model Manager Web), the DEM Orchestrator clears all the associated workflow instances to the nonfunctional DEM Worker, thus allowing the other DEM Workers to pick up the workflow. This explains why the failover process is seamless and fluid without the requirement of any extra steps for it to function during and after the failover.
If you have more than one vCenter endpoints, then you have to install an additional vSphere Agent. It's a 1:1 mapping between the vCenter endpoint and vSphere Agent. You can have multiple agents talking to the same vCenter endpoint in case of high availability but each agent should be on a different server, and they all should have the same name; otherwise, you will run into many issues. For example, if you have three agents...