What issues are you seeing, perhaps we can fix it for you without having to implement a Voice VLAN?
I would have to agree here, what issues are you having? Although having a voice VLAN is preferred, with properly configured QOS it is not necessary.
Can you give us an overview of your installation (controller, software version, number of device, trunking, network layout, etc) and what your issues are?
TBH, Mitel itself is very responsive to serious issues and if you are having significant problems that are not being addressed, the blame could fall back on your vendor. Not trying to nit-pick here, but Mitel equipment although far from perfect, is pretty rock stable in most instances.