Vinca has seed pods that look like little green bean pods. They form on the stems. They pod in pairs, that is two geen bean pods stuck together. As they start to mature the pods will separate into two single ones. They will brown and you can see the seeds through the pod. Check on them regularly as the pods will burst upon full maturity scattering their seeds. They are approximately 1" long. When you find the first ones, it's a piece of cake to harvest the rest.