If the pipe was leaking there could be no vacuum.
I don't know if you are arguing semantics or just being belligerent. True, there cannot be a perfect vacuum since the water would turn to vapor and occupy some of the space.
In the presence of a topside checkvalve, there most certainly can be a partial vacuum. The weight of the water falling back to the well would create it. It works on the same principle as a mercury barometer.
Fill a garden hose with water. Leave one end open and put your thumb over the other end and climb a ladder or stairs, dragging that end of the hose with you so that the other end remains lower. When you release your thumb at the top of the stairs, you will experience the suction. This same suction can suck in contaminated ground water.