bypass Unbound DNS for certain VLANs

[wotcha]

Part A: I would like 2 VLANs (VLAN 1 & VLAN 2) to use OPNsense's own Unbound. VLAN 1&2 are using a VPN gateway. 
Part B: I would like 2 other non-VPN VLANs (VLAN 3 & VLAN 4) to use another external DNS, for example, Cloudflare. Or whatever is set on the client's device.

I would also like optionally in both Part A & B if any of the clients set its own DNS on their device, for that external DNS to be honoured. If it is not possible in Part A, then definitely in Part B.

Can anyone point me in the right direction to do this?