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.
From what I have seen in the past from sources periwinkle does contain vinpocetine. It is in the seeds. But there is not enough to make it commercially viable. Therefore commercial vinpocetine is synthesized by tweaking the vincamine molecule to produce vinpocetine for the commercial market.
I'd recommend you to use the pure substance, because it's easier to monitor the right dosage.
Again I disagree. When people isolate a single compound from a plant they can alter the properties of the plant for one thing. If you isolate the coumarins from alfalfa you have a blood thinner when the whole plant is not due to the vitamin K.
Secondly, herbs have more than one active compound. Many times these active compounds work synergisitcally. If you only take vinpocetine then what about the other alkaloids with similar effects such as the vincamine and vinburnine?
I am curious do you eat isolates for food? In other words do you only take phenylalanine instead of eating a steak? Only take magnesium supplements instead of eating chlorophyll sources?................ If not then why? After all it would be so much easier to monitor the right dosage of nutrients. Or do you eat whole foods because the compounds in the food as nature made it works together?
If people want concentrated isolates that are not balanced then they might as well take pharmaceutical drugs since they are the same principle.