If I understand you correctly, you have several VoIP devices (eg PCs running softphones) and you want them ALL to be available on ports TCP 5060 and UDP RTP ports.  But when a packet arrives at the Vigor’s WAN interface port 5060, which of the devices on the LAN should the Vigor forward it to?  To all the devices on the LAN ? And if the Vigor could copy the packet to each device on the LAN, how would the users know who the VoIP call is for ?

This is not a DrayTek-specific issue; but is common to all routers and devices using NAT.  NAT requires one LAN IP address for each port being opened, so that it can know where to forward incoming packets to. If the router doesn’t have any NAT rule covering the incoming packet, the packet is simply ignored.

The traditional work-around is to open different port(s) for each device to be accessed from the WAN side (e.g. port 80 for the www server, port 81 for webcam, and port 8080 for router’s remote admin web interface – all using http: protocol).  This can be effective for a small number of devices and ports; but also requires that he WAN application can be configured to use different ports.

There are a couple of other NAT traversal mechanisms, and the one DrayTek prefers for SIP is STUN.  If your VoIP devices support STUN, then you do NOT need to open the ports, and instead configure each VoIP device to use a STUN server (stun.fwdnet.net is one that is freely available).

Another common NAT traversal technique supported by many VoIP providers is Outbound Proxy, which again is a setting at the VoIP devices.

Please check whether your VoIP devices and VoIP provider supports STUN and/or Outbound Proxy.  Alternatively, maybe you should investigate using an IP PBX internally; so that all incoming/outgoing calls are being trunked through one device (so only one set of ports need be opened).