Susan, this is not the 'Swamp Marigold', but the 'Wild Marigold'. It will reseed for sure in your garden. It starts blooming in spring, but since it self sows, you will have flowering plants until frost.
Your Vinca might be a cultivar, because most Vinca's bloom in spring and again in late summer.
Vinca doesn't easily set seed in colder climates, I also have problems with collecting seeds.
The seedpod looks like a small pea seedpod and contains only a few seeds.