What exactly is dumping syndrome

Gastric dumping syndrome, or rapid gastric emptying is a condition where ingested foods bypass the stomach too rapidly and enter the small intestine largely undigested.

Symptoms of early dumping include nausea, vomiting, bloating, cramping, diarrhea, dizziness, and fatigue.

http://en.wikipedia.org/wiki/Gastric_dumping_syndrome

because the pylorus is preserved, VSG people shouldn't "dump".

But a few VSG people who would argue that point. And it's generally fats and/or sugars that are the culprit. VSG used to be promoted as having no dumping syndrome. That seems to be gradually changing to "dumping syndrome does not occur or is minimal.".