My first thought was Vanilla planifolia, but the leaves of planifolia don't seem to have that line along the center.
Here's a planifolia...

The seller had a list of what he included as freebies for that particular month, and had Cappi cuttings amungst them but it doesn't resemble any Cappi that I'm familiar with and it was the only vine on the list for that month. It's really tempting to just email the seller and ask but he seemed pretty adamant about not inquiring so I'm going to try to ID it myself... but I'm stumped.
Here's one with my thumb for scale..

I've kept it in the shade with heavily filtered sunlight so far and it seems to have responded well to this, likes moisture and humidity, and roots very rapidly only taking a few days to grab dirt while the vertical growth is much slower in comparison which makes me think it is tropical-ish.
Also seems to be shooting up new growth from the roots a few inches away from the main stem.

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