Hi David,
There is no requirement that your VG350 needs to be on the voice VLAN. It can be in any network which has IP reachability to CUCM, phones, and the PSTN voice gateway.
That being said, a lot of people choose to put their VG gateways in a voice VLAN or the CUCM server VLAN, but you are not required to do so.
Hope this helps!