Hello Gr4cchus,
As daphnissov said, this is not how vSAN works.
Cache-tier and Capacity-tier play different roles - the former stores metadata relating to the data stored on the Disk-Group and is used for caching data (e.g. so that you don't have to wait for data to be written to slower devices for the IOs to be committed), the latter stores the actual data - thus you can't have a Disk-Group without either component as they serve different purposes and are dependent on one another.
Bob