The problem with compression within Vvols is that it's not of much use.

Of course there is many ways to compress or Dedup within a Storage Array.

If you have Vvols and don't have dedicated Storage (as the current trend is) you will have an Array Wide Inline compression as of my understanding.

Seeing this, you won't need Vvol specific compression since the compression lays over the whole FS.

This is how I understood it so far and I also had a talk with a friend of mine about this and he thinks the same.

I asked myself the same. I submitted an enhancement request and this is what dell/emc replied:

Thank you for submitting your request to support compression on vVOLs. Unfortunately we don’t have any plans to address this in Unity at this time since there is very low vVOL adoption. Our engineering teams are focused on delivering other critical features for the product. However, we will keep your request in our radar and will try to address this in future.

So, after questioning this whole thing again and talking to some friends and a few experts I finally found the solution to how/why you can't add Compression to selected vVols as you described it...

Ultimately your compression is laying upon the Storage system itself.

Your vVols are just logical restrictions which you lay over this. So you will have Disks which are compressed and disks which aren't.

The compression within the vVols will be possible if you lay the vVols over the compressed Storage via Storage Profiles i.g. Profile 1 says to only use Tier 2 and Tier 3 which are both compressed so you can have a fully compressed vVol but at the same time Tier 1 may not be compressed but Tier 3 is and you can lay your vVols over this with Storage Profile 2 so that it doesn't compress it in Tier 1 but then compresses it in Tier 3 for Speed purposes.