I'm not sure why you would need VLANs - seems like an unnecessary complication as you do have enough ports to do what you want.
I'm thinking that
you can't do this though until V7.5 arrives.
Maybe you could put a switch in front of the Astaro, then use two separate Ethernet ports for the two public IPs. That way you can at least masquerade each separate internal network to a separate public IP (interface).
I can't remember, but it also seems like there might be an issue with the HTTP Proxy.
I hope others can help.
Cheers - Bob