Two useful parts have arrived. One is a USB keyboard to serial adapter. The keyboard (and mouse) host looks like this:

It uses CH9350DS to do the dirty job of being a USB host to the mouse and keyboard and converts their output to scancode data on a serial port. Rather nice if true. The documentation is hard to decipher. Here's a link to a copy of the PDF: CH9350DS-Qinheng_KL-024-0000864.pdf There are DIP switches to select baud rate, also the function of two of them is to "set the state" whatever that means. There are only 32 combinations so it should be fine.

If the docs are to be believed, the output serial levels are 3V3 so that's nice.

The other part is a WM8978 audio codec. It's a neat little board (or an MP3 learning pole, apparently) that looks like this:

Unfortunately input and output jacks are not colour-coded. There's also a little microphone soldered from the opposite side for some reason. I don't really think it would be very useful in a vector-06c, but we'll see, maybe we can tape load over-the-air.

The codec chip is a bit more advanced than I need for this project. I could easily get away with a 1-bit sigma-delta DAC, but this board also provides with  a nice pair of 3.5mm jack connectors so it seemed like a nice thing to have. The datasheet is rather intimidating though. I've found a couple of projects with initial setup code that I hope would serve as a reference.

The datasheet link: https://www.mouser.com/datasheet/2/76/WM8978_v4.5-1141768.pdf

The pinout is labelled from the reverse side, but you can't see it fully because of the microphone, so here's the pinout picture:

It's also a rare case when a schematic is available, so here it is:

I have not yet tried any of these boards.