Answer:
The stopper at the top of the vial had to be completely sealed so that no gas could leak out of the vial and so that no water would be able to enter into the vial.
Explanation:
The vial needed to be completely sealed around the stopper because it insured that the measurement of the pressure in the test tube to be accurate. If it hadn't been tightly sealed, the air inside of the test tube would escape, causing the pressure of the oxygen to be impossible to be measured.
