What does the vent on top of the parachute do?

Answer 1

Firstly there are two types of parachutes, the old fashion "roundies" and the modern ram air square chutes.

I presume that the hole in the top of round parachutes creates an airflow through the canopy during deployment (opening) which reduces the chances of a malfunction during this critical phase. If not for this airflow, the flow of air around the unopened canopy would create an air pressure differential between the inside and outside of the fabric and the resultant suction would tend to keep the canopy in a collapsed state, not a good thing to happen as the ground rushes up at you.

It would also make the canopy more steerable.

The holes in the front of a square canopy are sealed at the back of the chute and allow the chute to pressurise into its aerodynamic shape.