Had a customer ask me today if he could mirror (or span) a whole VLAN as opposed to just specific ports. Documentation is pretty clear that you can mirror specific ports or a range of ports on the same switch (or switch stack). Looking for confirmation that currently there is no capability to mirror a whole VLAN. Thanks!
My assumptions are the same as your's only port mirroring is currently available.
At the moment, VLAN mirroring is not possible in Cisco Meraki devices. However, I would definitely encourage you to make a feature request under the "Make I wish" section in Dashboard.
please request this feature under "Make a Wish" section, I have done it and I'm hoping for this feature to be available soon, we desperately need this and we are forced to run legacy devices in order to achieve this.