Adding to what Adam said, RMAN backups of a VDB are supported, but it is comparatively heavy lifting which need not be done when a snapshot using the Delphix Engine interface would do basically the same job.
If you intend on using RMAN the only advice would be to steer clear of partial recovery, e.g. say: RMAN> recover database until logseq 100;
When you start the database you will need to follow on with: "alter database open resetlogs".
This will change the database incarnation number making it different from the value we hold in the Delphix engine, effectively acting like a rewind, but not telling the Engine about it. This will break things because we won't know which snapshot is the parent; so partial recovery is not something we would support. (Rewinding the VDB prior to the restore would bring you back out of this scenario).