Possibly Pentalinon luteum?

The leaves and stem look fatter (more succulent) and leaves are less shiny than Pentalinon luteum I see online. Certainly not like red or pink dipladenias for sure. No flowers or any of them but these seem like young plants so there is that. The presentation on a circular trellis reminded me of stephanotis but the color and size of leaves was off (too light and small). Also, the texture tells me these leaves and stems had way more water than a stephanotis or a Hoya, which was my second guess. Anyhow, I left them behind. No space for mystery plants on my windowsills.

especially something claiming to be a 'tropical vine' (even in my own, admittedly tiny garden).

Could it be hoya cumingiana?

I agree with GetGoing - Hoya.

H. cumingiana is most likely, but there are a couple of others with a similar look.

As long as it is a hoya, it might be welcome. I have only one hoya, carnosa krimson princess. It was small but for six bucks and a nice small pot and trellis (though I can't imagine what would be happy on that tiny trellis), I am tempted.

I brought one home for $10 and then looked up what plants were being sold online by the company whose tag was on the pot and it turns out they are selling the Cumingiana in that circular trellis presentation, among other tropical plants.

Looks like my stephanotis