The softphone will be on the VLAN for DATA because the pc is on that VLAN. You will need a router to get between the VLANs. The Your Assistant server will be on the VOICE VLAN and the Your Assistant clients will be on the PCs.
I find that the best way to design the VLANs is:
1) Switch ports that have the Mitel IP phones on them, have access to both VLANs. Make the DATA VLAN as default or native and untagged. Tag the VOICE VLAN for these ports.
2) The switch port that the Mitel VoIP system is on to only have access to the VOICE VLAN. This is so the internal DHCP server in the Mitel system will not confict with your DATA VLAN DHCP server. Also set the port for the Your Assistant Server to only have access to the VOICE VLAN.
3) Any device on the Data VLAN that provides DHCP, only have access to the DATA VLAN. For the same reason as above.
4) Have a router that has access to both VLANs, unless your Cisco Switch is layer 3 capable.
5) I would recommend that you statically assign each Mitel IP set to have a VLAN ID that is your VOICE VLAN. That way when the set boots up, it will send its DHCP request only to your Mitel system. There is a what to set up the DATA DHCP server to do this for you if you want to go through that.
Good Luck!