Good question - you're working with HMC CLI (chhwres) on PowerVM.

The tricky part is that chhwres attributes differ depending on the resource subtype (--rsubtype), and IBM doesn't publish them in one single manpage, but in the HMC CLI command reference (part of IBM Knowledge Center / PowerVM InfoCenter).

Where to find the attributes

For your case:

Command: chhwres -r virtualio -m <server> --rsubtype vnic ...

The full list of attributes for --rsubtype vnic is in the PowerVM HMC CLI Command Reference under chhwres → virtualio → vnic.

The documentation is in IBM Knowledge Center, section "Virtual NIC Resource Attributes".

About VLANs on vNICs

The error you saw:

HSCL1327 A port VLAN ID is not allowed with the specified VLAN restriction.

means you're mixing two different VLAN configuration modes:

port_vlan_id → used for untagged/native VLAN assignment.

allowed_vlans / vlan_ids / vlan_tag_ids → used for VLAN trunking (tagged VLANs).

The attribute controlling the restriction is typically:

allowed_vlan_ids (sometimes just vlan_ids in older docs).

So:

If you want a single access VLAN: use port_vlan_id=6.

If you want tagging/trunking: instead specify allowed_vlan_ids=6 (or a comma-separated list/range like 1-10,20,30), without port_vlan_id.

Example

Assign vNIC with VLAN tagging (trunk VLAN 6) to an LPAR:

chhwres -r virtualio -m <Server> --rsubtype vnic -o a -p <LPAR> \

  -a "backing_devices=sriov/vio1/1/1/0/2.0,allowed_vlan_ids=6"

If you want VLAN 6 as native/untagged plus allow tagging on others, you can combine:

-a "backing_devices=...,port_vlan_id=6,allowed_vlan_ids=6,10-20"

✅ Answer to your question:

The attribute you're looking for is allowed_vlan_ids (sometimes referred to as VLAN restriction in the docs).