It is a kind of straight long vine.

Geographic location?

Looks like Vinca major.

Not Vinca major.

Why do you think it isn't Vinca major?

Leaves are too small and of the wrong shape for Vinca major.

If it had darker stems, I'd guess wire vine, Muehlenbeckia axillaris.

We have nothing to show scale and we can't see the top surfaces of the leaves. Also, as Embothrium said, we have no idea where in the word this is growing.

It can't be Muehlenbeckia because the foliage is opposite. To be honest Vinca was my first thought too. But I'm given pause for thought by features such as the apparent softness of the leaves and stems and what appear to be twining petioles.

Do you have a picture of the plant in situ?

Geographical location : central Greece, Pelion mountain, 800 meters altitude, wildlife. Unfortunately, I didn't think of taking a photo of the plant because it came from above a high rock wall amid many other plants and because I thought it would be easy to identify.

Also, leaves are soft and opposite each other at pairs and with quite a distance between pairs. I don't know if it helps.

Almost certainly it will not survive.

Is this not variegated Vinca major? I still think that's what it is, just plain green kind. The softness of the leaves and distance between them is likely a function of growing in the shade.