In the past five years, there have been a number of small vines coming out of South America, mostly from Ecuador. This is the third of this type that I keep and is distinguished by the three-point serration on each leaf. The two other types are very similar, bright green and very small leaves, one from Peru and a second Ecuadorian species. Overall it seems to be a rather slow-growing group of plants, but once acclimated they can put out moderate growth each year. They seem to require a vertical surface to climb. Otherwise, they will not put out much new growth and can stagnate for years at a time. This one was on sphagnum for years and hardly grew, but is now finally taking off on a tree branch. I think they would do best if treated similar to orchids.
As for the true genus of this plant, it remains a mystery! Hydrangea has not been discounted as far as I know, so for referencing I am putting that for now. If anyone has a better guess, please comment below or contact me!