I have Private Internet Access on my download server (virtual machine). I want to be able to use port forwarding so that I can still use Remote Transmission on my phone to manage my downloads while away from home.
So -
I usually log in with my own ip on the default port - so say 123.123.123.123:9091
When I port forward PIA it tells me eg 456.456.456.456:51000
When I set up transmision then do I switch the port to the 51000? what about my router - to I set my incoming port as 51000 to 9091??
If you are connecting to your device from the other end of a VPN tunnel then you shouldn't need to worry about your routers ports, however I don't think that is what you will actually do. The PIA NIC is a virtual NIC for that VPN and the VM's main NIC would still have it's own internal IP address on your home network. Therefore you just log in to your own IP on your usual port and set the routers port forwarding to go to your VM's main NIC, not the PIA VPN NIC.
I could be wrong in which case listen to whoever comes along and knows this stuff :-D
unless your trying to contain a VM, because you don't want it to access other computers in your subnet, I think it's better to bridge. Then your DHCP server, your router, allocates the VM a standard IP address, and you can even give it a static IP address based on it's random mac address.
Port forwarding sort of seems the wrong way to go. VPN is a good thing to try to implement, on the head of your router, so there might be port forwarding, depending on what hardware your router is. If you had a router that could allocate IPs and handle VPN, than the VPN would more easily drop VPN (remote) clients into the, and thus have access to your torrenting VM.
Might want to consider a DNS server on your local network, so you can give that box a unique handle. In my experience avahi, bonjour, or zeroconfig, doesn't work through VPN.