Periwinkle actually contains several brain enhancers including vinpocetine and vincamine.
The dosage of periwinkle will depend on the form you are taking. The powder is very difficult to find, which is why I have to buy such large quantities for my my memory formula. It is a special order so I cannot buy a pound or two. On the general market I have only seen it sold cut or tinctured. And both the common periwinkle (V. minor) and Madagascar periwinkle (V. rosea, C. Rosea) are offered in tinctures. You want common periwinkle, not Madagascar periwinkle. Madagascar periwinkle is the source for the chemotherapy drugs vincristine and vinblastine.
If you go with the vinpocetine capsules then I recommend 1 capsule 3 times daily.
Regardless of which one you go with you need to take them with herbs to hold moisture in the intestines since the alkaloids fry up the intestinal secretions leading to constipation. Good choices are seaweeds, marshmallow root, slippery elm or yucca root.