Thanks for the reply, but that's not how I understand it. If you put the ADSL router into half bridge mode, as many do, it transparently handles the PPPoE session and bridges traffic over that session to the physically attached device (LAN router or PC). Specifically (I imagine) to the port on which it received the DHCP request. Subject to certain limitations (*) that allows the attached device to function as if it had a direct connection to the internet, receiving a public IP through DHCP, and having no knowledge of nor involvement in PPPoE.
If the device is a PC running vbox, the NIC through which it is attached to the (now transparent) ADSL router is just like any other NIC, and should be able to be bridged to a VM, allowing the VM to behave like it owns the physical NIC and has a direct connection to the WAN (internet). Which is what I want to achieve. I used to do that all the time when I had cable, before I switched ISPs to an ADSL one.
And I understand it's quite common to use more than one physical connection to the ADSL router, for a specific purpose - many ADSL routers (in bridged or half bridged modes) still do routing, and that allows you to make a second physical connection between the ADSL router and internal device (PC or LAN router) to access its internal management pages. You typically want to do that to retain the ability to see your ADSL link status, for example, but you can't typically do that over your primary device-ADSL router connection any more, because the latter is now blindly and transparently bridging the former's traffic to the internet (over its internal PPPoE session). So it's a common technique to put a second NIC in your PC (or use a second port on your LAN router), on a different subnet (the one on which the ADSL routers statically configured LAN IP address resides), and attach it to another port on your (bridged or half bridged) ADSL router for that purpose.
(*) which can be deal breaking. One is that some LAN routers can't handle being DHCP-ed an IP address which is on a different subnet to the default gateway. Another is that this doesn't support multiple IPs, if you have that from your ISP. A third, and more typically encountered, problem is connection interruptions when your ISP changes your IP, but you can work around that using short DHCP lease times between the ADSL router and PC (or LAN router). For these reasons, some consider using half bridged mode on the ADSL router to be the least desirable solution. it probably depends on your specific hardware and your ISP.
Also, TBH, I still don't see why, in principle, a PPPoE "logical network adapter" on the PC, as you have when running the ADSL router in full bridged mode, couldn't be directly bridged to something else (such as a vbox virtual NIC). It's still a network port that has an IP address and TCP/IP binding that can be filtered, after all. It's my experience that Windows' way of doing networking creates the limitations, which Linux doesn't suffer from. I haven't looked at it yet, but I'll be genuinely surprised if Linux doesn't let me create a PPPoE "logical NIC" and bridge it to something else. We'll see!