I have been searching the web all morning on how to harvest my plants for seeds and i can not find info on vinca seeds
If you are referring to annual Vinca, Catharanthus roseus, you will find the seed pods along the stem under the current blossoms, similar to the way Cleome species form elongated seedpods if you have ever collected those.
The seeds are in slender, cigar shaped cylinders that start out green and turn brown and split to disperse black seeds. You may have to keep an eye out so that you can harvest as soon as they brown up. Otherwise the seedpods will split and you will miss the opportunity to collect the seeds. They reliably self sow in SE Kentucky.
I have searched the web for this info. Finally, you gave me the answer. I sure would love some good photos of the seed pods to know what to look for.
From what I remember, the pods show up fairly late in the season. These reseed in the black asphalt cracks at a nearby gov't bldg. how they survive and bloom in that heat, I'll never know.