Its true Rene Caisse went with the four main herbs starting in the 1920s. I don't know if she used periwinkle after that, and we don't use it in our tea at this point, but when we finally have the proper herbs to produce the original 8-herb formula (by growing them, since goldthread, sheep sorrel root, red clover root, watercress root, and periwinkle root are not commercially available at this time, anywhere), it will be.
Mali Klein has been experimenting with the original 8 herb formula, and uses the vinca major root. She has had very promising results, indicating that this 8 herb formula is very strong medicine indeed. It hasn't been used since 1926. The beginning of her research and writing on this original formula is what the Black Root Medicine
book is all about. Mali cites some research on periwinkle in her book, there are 7 references in the index. A quick look at one of them indicates that periwinkle mediates the properties in goldthread that lead to contraindication for folks with high blood pressure.
Hope this was helpful! Both Mali Klein and I are dedicated to growing the herbs and working with the original formula more in the future.