Using a VM as networking pass-through / isolating a VPN app

[MammothMountain]

Apologies if this is the wrong place to ask, I'm only here because the subforums are divided among host OS'es. The guests are all Debian 12.

I want to have a VM connect to a VPN app which is isolated in another VM. Basically I'm trying to use the second VM as a networking passthru. I want several VM's on this VPN, but not all of them nor the host OS -- I figured this was the easiest way to do it. This also sandboxes the VPN app.

How do I go about this? I understand I need to add all the VMs on the same host-only network and also give the passthru VM a NAT adapter, but I'm not sure how to configure the passthru VM to make it pass the traffic through. Do I need a third VM running ddwrt or something?

[BillG]

That is really a networking problem, not a VirtualBox one. Once you have a clear picture in your head (or a network diagram) the implementation is the same, not matter what devices you use to implement it (physical or virtual). The networking protocols don't know or care what the devices are.

Work out how all your vms in general will access the network, work out what needs to be different for the vms which need access to the VPN, and it should all be clear how to configure the devices.