Cyphostemma (Cissus) species are short day plant. They germinate at 25-30 °C and in my experience very unevenly. Sometimes I get no results sowing them the first time and I re-use the soil for other plants. Then suddenly the next year in spring everywhere Cissus seedlings start to pop up in those containers. This makes me believe that these species need a period of dormancy or a high fluctuation in temperatures in order to trigger germination. You could try to keep the seeds in the fridge for a couple of days or longer and then sow them at warm temperature.