Nat – VirtualBox – Static Guest, accessible from host

There are several ways you can approach this:

• Don't use a NAT adapter. I realize this may not be possible/desirable, but if you can select "Bridged" adapter and get an IP from that same subnet, all these problems (both gust->host and host->guest) go away.

• Use two virtual adapters, the regular NAT one for internet access, and a second "loopback" adapter in bridged mode on your own private subnet. The loopback will be used for communication between the host and guest(s), while the NAT adapter will handle the traffic outside of the host.

• There are a number of tricks to do port forwarding to the hosts, as well as open up guests to see the host. By default, I believe, both of these are blocked as to provide a better sandbox. There are plenty of questions here on Server Fault (tagged with VirtualBox) that talk about punching holes as described.

You can probably do it, but I would recommend against it.

The way to go about this is to have the VM establish it's own VPN connection to the target server/network.

Now, if you really want to do this, you would need to create another bridge on the host, then create the tunnel interface as a static interface (i.e. not created by the VPN when it starts up) and add it to the bridge. Only then does this tunnel become available to the virtual machine. VirtualBox can then be instructed to virtualize this interface, which makes it available to the VM.
The part I am unsure of is whether or not a tunnel can be created outside of the VPN client and then be made available to the client for usage. You might have to look to tap devices instead of tun devices.
This can get rather complicated and will result in a situation where your VM client may not have any networking if the VPN isn't up (unless you also virtualize the other bridge). As said, I would recommend against it.