Hello,
Virtualbox is very disturbing with network configuration. Their basic strategy is to create isolated networks to test without risking to transmit to the extwrnal world.
Virtual networks are like virtual machines, isolated.
The second wrong assessment is to suppose a symetry in the device configuration. It is closer to reality to view the network device as unidirectional.
The specific topology, that you want to build, is achived by deploying multiple devices. Each device is managing one of the route you want.
Now, just review the virtualbox documentation with this new perspective and you will be able to build any kind of topology.
If you want a ready made config with a proxmox virtualization in virtualbox, you can use mine but it is less fun.
The proxmox is virtualized in virtualbox with fix local ip adress and internet connection. Deployed proxmox Vm also have access to internet. Remote access to the proxmox hypervisor could be achieved by ssh port forwarding. Remote access to local proxmox node could be achieved with vagrant remore access.
You will find my full documentation
On github.com user: pcolot01 project: /vagrant-proxmox file: /blob/master/README.md
Best regards
Pierre
