If you Google the plant name you will find a bunch of web sites and perhaps the answer to your question. I know that they are related to Cyphostemas and their fruits are considered unpalatable due to high oxalic acid content.

They are also related to grapes!

Good point Jo, I neglected to mention that. Who wants to try one first???

I can tell you with authority that Cyphostemma berries have a quite unpleasant taste. I'd think that Cissus aren't much different.

couldn't find this specific one but the other cissus I looked up say they cause contact dermatitis and "symptomatic ingestion", which usually means an upset stomach. So not toxic but probably unpleasant to eat.
Tally HO!