I actually waited for your bootChange to see if you used the 0x0 flag
The vlan_off makes sense. It all has to do with your switch config.
If you set the tagging to the port being untagged in VLAN 18 and set the other VLAN's to FORBID or NO, you need to default the VLAN in IP configuration in the Mitel to 1. In other words with an untagged setting you let the switches define the VLAN which is imo recommended and if you set it to tagged you can define it through the Mitel 3300 in the IP configuration menu.
I never use tagged for data or others on the port where the Mitel 3300 is connected, since IP routing wether set by the switch or, if you use a static route to your firewall, your firewall defines if you can reach the voice parts from the data network. Out of my head, No if they dont need access and I think LACP, if used in your network, requires a forbid setting.
I hope this info clearifies.