I could be off, since I haven't used it the way @LabRat has, but from my understanding of it the "render" card is the one you need to worry about the VRAM on as you use the raster performance from that card (or even RT performance). The AFMF is processed, pretty sure by one of the secondary "cores" or whatever on the AMD cards, like the encoding or decoding section of the card. So no, it doesn't need VRAM, it just needs to be connected to your display with AFMF on, and that's it. Select the other card as your render card (Windows 11 has that capability). And that's it. You're now rendering on a GTX 1080Ti with AFMF on through the RX 6400 for example.
And yes, it really would be a cheap way for ANYONE really to add AFMF to their system. If your current GPU is more powerful than an RX 6400 (to get the AFMF features on), then it'd be worth it to try. It would be "pointless" to say put a 6900XT in a system with a 1080, as the 6900XT is more powerful already. However it "would work" in that case too. That's also why it doesn't matter if you use a 6900XT or a 6400, you're not using the raster performance of the card at all, nor its RT capabilities.
Think of it kind of like back in the day when we'd add a dedicated GT240 or 8800GT or something to our big HD4890 / HD5870 / HD6970 rigs for the PhysX support. Same idea here.
Right now, I believe the beta driver is only for RX 6000 - 7000 series GPU's. It probably will roll out later on to the APU's though, I don't see why they'd keep AFMF off of those, those need it more than a 7900XTX does. They might even roll it out (way later) to older cards like the RX 5000 series and maybe Polaris too (all currently supported cards). If they don't, I suspect the guys behind the Amernizone drivers probably will.