Hi I have a Dufour 31 which is the previous version of your boat. Both vents are unnecessary. The one terminating near the engine is indeed an air inlet but is wholly unnecessary. So long as the stern vent is not obstructed the air will find its own way to the engine. The fan is there to extract hot air from the engine. It is completely useless the engine itself extracts more air and expels it outside through the exhaust. I had my engine replaced several years ago and the person doing the installation simply removed it.