That is a very interesting chip!
I found them on mouser as well for around 4-5 USD a piece and they come in DIY solder friendly packages!
I'm currently looking at the datasheet for the
VNC2-32L1CAnd it basically is a Microcontroller with USB Support.
So it has it's own CPU, RAM, ROM, and a bunch of IO. so you use that to handle the actual USB Data and then communicate with the main system via the IO (UART, SPI, FIFO, etc).
so in a 65xx System you could use a 65SPI Core or direct 8-bit Interface to connect to the IC, you just need to use some of the existing firmware or design your own (something more optimized for a 65xx Interface) to handle the commands and data transfer.
one downside of the chip is that (like most/all Microcontrollers) it's not programmed out of the factory... so you need to program it yourself using FTDI's proprietary programmer (~20 USD on their site, Mouser, and DigiKey) and their software (though it doesn't appear to require any kind of licence).
plus it requires 3.3V on it's Vcc pins to run and that's also the maximum voltage of the IO, but the datasheet does mention that it has 5V tolerant inputs on the GPIO pins...
so in theory you could have a 5V Unidirectional Interface (like SPI or a UART) hooked up to it without level shifters as long as the voltage it outputs is high enough for 5V logic to recognize as a stable "1".
but i'm kinda wondering how this would compare to something more mainstream, like an STM32 with built in USB support. the STM32 definitely has a more established user base, and the chips are similarly priced for much more CPU power and IO.
though i guess the main goal of the VNC2 is kind of the opposite of a regular Microcontroller, instead of a high speed/capacity CPU/RAM/ROM Combo surrounded by IO, it's more of a main IO Controller + a few smaller ones duct taped to a tiny and custom CPU/RAM/ROM Combo.