mtu field is not exposed over the API for VLAN

You can set and update the MTU for a VLAN in MAAS 1.9, but that value is not shown over the API.

ubuntu@maas-bcache:~$ maas admin vlans read 0
Success.
Machine-readable output follows:
[
    {
        "name": "untagged",
        "resource_uri": "/MAAS/api/1.0/vlans/0/",
        "fabric": "fabric-0",
        "vid": 0,
        "id": 0
    },
    {
        "name": "Management",
        "resource_uri": "/MAAS/api/1.0/vlans/5001/",
        "fabric": "fabric-0",
        "vid": 10,
        "id": 5001
    }
]