periwinkle extract doesn`t contains vinpocetine. That plant is a source of vincamine, which is the precursor of vinpocetine, and it is necessary a chemical processing to obtain vinpocetine. There's only a plant (Cryoceras longiflorus) which contains a little amount of vinpocetine.
So, if you need to improve cerebral blood flow, you should use pure vinpocetine. However, if you only need a peripheral vasodilator you could use pure vincamine or periwinkle extract.
I'd recommend you to use the pure substance, because it's easier to monitor the right dosage.