Bizarre and annoying coda number 2: apparently the binding of “virtual interface” to “network device” on macOS is by like… vendor, or driver type, or USB device name, or something, and *not* by MAC addresses. 2 of my adapters are Realtek and 1 is Intel. When I added the VLAN to the Intel adapter, it applied to the Intel adapter. when I added one to the first realtek, it applied to both realteks.
This doesn’t cause any problems for me in this specific situation, but… what an odd choice