How can this be accomplished, it refuses to let me add it in the web interface?

are you trying to forward to another network that passes through a router to get to your network?

my internal switches have two vlans, one for data, which the draytek resides, and a voice network. the switch does the internal routing, and the draytek has a static route set in place to point the voice vlan towards the switch. It just wont allow me to add the other network in the port forwarding form

I have seen this too and to be honest it's to be expected! port forwarding is a NAT feature and if that NAT is translating its own private subnet then that is all you should be able to forward a port to... only way I have gotten around that is to place the other VLAN device on the NAT'd subnet using a second NAT device and port forward via two NAT port forward rules, one for each NAT, each managing its own subnet.

there is a second subnet option in most draytek products that might be of some use but I've had no experience of it, so cannot comment on its proper usage. Just guessing... maybe you could set the second subnet in the LAN General subnet to match the VLAN subnet... might do something usefull or not, as I said, I've not had cause to explore its function or capabilities.