Neither - it isn't Vinca major nor minor. It's Catharanthus roseus, an entirely different species confusingly know as Madagascar Periwinkle or 'Vinca' in common parlance.

This post was edited by floral_uk on Sun, Aug 3, 14 at 9:09

Catharanthus roseus not only gets confused for the vincas, but because of that might be sited in the wrong location. C. roseus prefers full sun, arid conditions and will rot if kept too shady and/or moist.

They are a reliable (non-irrigated) parking lot island plant, for example.

Thanks for clearing up my confusion :) I will read up more on the care of this plant. It is in a good sunny location currently and I try not to overwater it. Its shop tag read "Periwinkle" but I couldn't tally it with the pictures of vinca major/minor I saw online. So now I know the correct name, thanks to you people.