Manual VLAN selection from the phone itself isn’t currently an option. The phone defaults to LLDP for VLAN assignment. You can provision the phone to manually attach to a VLAN inside of Switchvox by modifying the VLAN settings for the Phone Network:
Admin Tool > Server > Networking > Phone Networks > Edit > Advanced Settings > Virtual LAN Settings > Discovery Mode > Manual > (enter ID)
If you’re in DPMA, then you’d create a phone network (https://wiki.asterisk.org/wiki/display/DIGIUM/DPMA+Configuration#DPMAConfiguration-NetworkConfigurationOptions) with network_vlan_discovery_mode set to MANUAL, and then network_vlan_id to the ID of the VLAN on which you want the phone.
As it is a commercial product, Digium’s Support department (accessible via the means outlined at http://www.digium.com/support) is always your best stop.
As noted before for the user who originally posted this thread, you’ll want to be on 1.3.x firmware to resolve known issues with application of config elements while attached to a VLAN.