Hi Vinca!
It would have been vinca seeds that fell last year. A vinca seed is like poppy seeds or pepper flakes, hardly noticeable. This year after they have flowering and some of the blooms have faded. You can look and collect seeds. Take white paper and gently shake the vinca. Seeds should fall into the paper. Keep repeating the process every two weeks or so. At the end of the season shake the plants out on the ground. Vinca from seed takes a little longer to grow because vinca likes soil temps in the 70s. May you be blessed with a safe and wonderful weekend!