What makes vacuum fried food retain and concentrate the food's flavor?
Because you are frying at a lower temperature, the food is closer to it's natural state (no browning) and is less cooked.  That coupled with the significant drop in moisture, you are concentrating the flavor of the food (just like syrup, only with solid food).  It's also typically done with food that is more susceptable to burning, foods with higher sugar contents, so you are often left with a sweeter product.