How can a vSphere administrator ensure that a set of virtual machines run on different hosts in a DRS cluster?

A vApp is a container for grouping VMs to manage their startup order and shared networking/storage properties, and provides no host placement enforcement or VM-to-VM separation capability.

A VM-Host affinity rule controls which hosts a specific VM is allowed or required to run on, but does not enforce separation between two VMs - multiple VMs with identical VM-Host affinity rules can still be co-located on the same host.

Folders in the vSphere inventory are a purely organizational construct for the UI and have no effect on DRS scheduling, host assignment, or VM placement decisions.

A VM-VM affinity rule configured with the 'separate' (anti-affinity) option directs DRS to ensure the listed VMs never share the same physical host. DRS enforces this rule during both initial placement and ongoing load balancing, issuing vMotion migrations as needed to maintain the required host separation between those VMs.