Hi Aomar, the simple answer is that you should enable VLAN's on the PMP450 equipment if you want those VLAN's to exist/extend down to the PMP450 AP and SM. Some users put customer's data on a different VLAN per region, per individual or per application type (i.e. VoIP traffic), but also want an internal management VLAN that is separate.
If you enable VLAN's and set a management ID, the GUI will only be accessible when accessing from that VLAN (or via the AP proxy). This isn't too much of an issue for the AP since it should be on the network and reachable. However, it presents a problem for SM's, especially during the install process, if your installers don't have a computer on the management VLAN.
I would recommend using our cnArcher App to install SM's (from out of the box). That will allow you to access the SM and do the alignment, as well as, set the VLAN as a final step.
Here are some resources that can help with the VLAN question: