Hello-
Well, depending on your desired outcome -- if you wanted all VMs and to only display datastore names for certain datastores, you would put such a filter inside of the calculated property for Datastore.
More likely, I would think, is that you are wanting to just get info about VMs that are on given datastores. If that is the case, you can do the filtering by just getting the desired datastore(s) first, and then get the VMs that reside on them and do the Select-Object statement that you already have built. So, for this, you would just put a Get-Datastore call at the start with the given datastore(s) name wildcard, and then pipe that to the rest of what you already have, like:
Get-Datastore-NamemyDatastores* | Get-VM | Select ...
How about that?