@UpperBayGardener (zone 7) I suspect it would need some extraordinary care here, such as:

* insulating the sidewalls of a raised bed

* using a raised bed to prevent standing water around the roots, which would quickly lower in temperature to the air temperature

* thick mulch layer to slow the rate at which warmer air in the soil escapes

Unless provided a very unique and specific microclimate (none of which you have described), it simply won't survive winter in a zone 9b Bay area garden.

For kicks I asked this question of 'The Horticulturalists' on Youtube. I wouldn't exactly call Melbourne a milder climate than northern California (though some would...) but it's definitely more favorable for warm climate subtropical plants. The downtown is 10b - though the winters are roughly as cool as the Bay Area, a big difference is they are drier: Melbourne's modest rainfall peaks are in spring and fall. This could make a difference, especially for a plant from winter dry SE Asia. Zone 10 subtropical highland plants that need mild summers like Luculia easily grow in Melbourne, as they do in Berkeley. Having visited both of the RBGVsites, I'd say it looks as "tropical" as anything in the Bay Area, with maybe the exception of not quite having so many weird Mesoamerican cloud forest things like daisy trees.

Well even there, they consider it a houseplant! So based on that, I'd advise there's absolutely no hope of growing it anywhere in Northern California. This got me thinking about Southern California and I remembered San Marcos Grower's website is still up, for now, even though the nursery is sadly closing soon. Even they never grew it in the many decades they grew subtropical plants for the LA climate! Although they did produce a few other tender Cissus. So, sorry, definitely too much of a stretch for you.

https://www.smgrowers.com/search/basesearch.asp?strSearchText=cissus&x=0&y=0

@UpperBayGardener (zone 7) It's probably hopeless, but I will try adding a starter plant into a raised bed and let's see if it can even come back in the Spring.

Okay, I put a starter plant into a small raised bed. It may not come back in Spring, but let's find out.

One thing that confuses me about Cissus discolor: I saw someone replant one prior to its Spring bloom, and it was basically a group of tubers, like miniature potatoes. The roots and stems shoot out from those. Why would a tuber have problems surviving a zone 9b Winter?

Are you sure about the tubers? I had not heard of that and I couldn't find any references to or illustrations of tubers.

As to whether tubers would survive the winter, many tropical tubers cannot take cool or wet soil conditions. Just being a tuber doesn't make them invincible.

@floral_uk z.8/9 SW UK Ah, my bad. The plant video I saw was for Dioscorea discolor:

https://www.youtube.com/watch?v=8cnE6nG3lig

And I confirm that the tubers of that plant won't survive Winter moisture either.

So, bottom line, my Cissus is likely to die over the Winter, but it was a cheap experiment to learn a bit more about the plant.