Interesting find.. the problem with most vector ports to the 64, were that the hardware really just wasn't fast enough to make a great experience in the port... Trying to play star wars on a 64 is painful, especially once you have a good number of objects being rendered on the screen... The 128 does for faster processing and in 80 columns higher fidelity graphics... but pushing bits around in the VDC is not exactly quick....
But if someone is dedicated, this base project could be a decent starting point.
Sorry for reviving an old thread, but TEMPEST is one of my favorites. I've had many ideas to improve my "proof of concept" but -- as you might imagine -- I've been totally distracted from anything CBM-related due to global pandemic = Covid-19. However, I wanted to share with you some videos I found about this arcade classic and retro-engineering...
Part 5 [VAPORWARE = Promised in part 4 but never published!]
Although "Dave", a Micro$oft employee, explains many things, none of them are C128-specific. But he explains the general idea of reverse-engineering TEMPEST much like I did for the C128.
Anyway, I hope you find his videos informative or entertaining. With any luck I'll be able to produce a version of TEMPEST that runs in real time on C128... maybe even C64...