There is confusion caused by the use of the names Vinca and periwinkle for two different plants. They are both members of the Apocynaceae family, but require different cultivation. While you specified Vinca, some experts list Madagascar Periwinkle (Catharanthus roseus) under the Vinca name which should more properly be used as the genus name for the other species.

I am in Central Texas and have only had success with Vinca major. That success has been limited by drought and excessive heat, though it persists. I tried Vinca minor years ago, but it never grew well and died out in a couple of years. Both of the above prefer part shade. Catharanthus roseus, however, grows very well in full sun, heat and drought. C. roseus is a tropical perennial grown as an annual in most of the U.S.

Park's book Success With Seeds recommends Catharanthus roseus be started indoors 12 weeks before the last frost date, maintaining a temperature within the planting medium of 70-75ºF. Germination takes 15-20 days, and the seed must be completely covered as it needs darkness to germinate. This does not mean you can't start the seed outdoors where they are to grow as long as the soil temperature is warm enough. It's just easier to control conditions inside during their longer time to germination and emergence.

Vinca species are slow growing from seed, and pretty much always started from rooted cuttings or divisions. I can't find any info on growing this from seed.

I'm planting the Madagascar Periwinkle seeds this time. I have these seed pots indoors (73º) in a room whose blinds are closed most of the day, but as they are plastic miniblinds, they do let a little light in. When they germinate/emerge I'll open the blinds up fully, as the windows don't get direct sun. The last Madagascars I had in pots out front in full sun most of the day lasted 3 years but they succumbed to our arctic freeze this year (I'm in Temple).

I do have the big-leaf Vinca Major (planted in-ground by previous owner) in a bed out front. It came through the snow/ice freeze beautifully. The ground ivy has moved in amongst it now, also planted by previous owner. :)

As long as the seeds are covered with soil they should sprout with the proper soil temperature. When this book says a seed needs darkness they mean NO light. When the book says a seed needs light to germinate it's best to surface sow it and water with a fine mist. I've found this book to be very helpful over the years. Good luck.