I've never noticed it, Assaf. I've always wondered about the effect of a generic vented hood on its own, too. But I've never bothered to test for it.
Actually, I have sometimes been bothered to test for this and that. But I always forget what I'd tested for and just end up, when I come to develop months later, with a series of indistinguishable pictures of my sitting room.
I have a straight-sided vented hood where the vent lines up with the viewfinder, and a sloping-sided hood where it doesn't. Don't know whether this is my luck or the manufacturer's judgement.