The vents are in the picture on the back of the boat.
And what happens they act like a vacume and hold the
back of the boat down, when they made them bigger
it let more air in the holes in the back of the boat and vented
out the hole under the boat in the last pictue you can see the holes
above the propshaft. The air is sucked backwards. Hope it helps don't know if I explained it correct.

I could be wrong but thought they are used to vent air to the props when taking off to reduce the bow lift?

Yes you are right. And has to do with a vacume
on holdIng the ass down. I will let the pros
describe it better.I'm just learning about this myself.

Vents are simply running a hole (fitting) with a hose on the bottom of the boat to somewhere above the water line to release the vaccum of the water sticking to the bottom of the boat in a place it should not be. I usually run the hoses inside the boat hiding yet above the water line. I have heard of some race boats that have experimented with vents in the notches of the step bottoms.

In the first post the third photo there is a very large vent hole in the top of the rather large pocket at the transom. That pocket can be very diffucult to over come as it fills with water and wants to stick the hull to the water surface while planning off. Venting cures that.