well with the kind i'm thinking of it would take longer to explain than to make... i'd take a cylinder, select half the verts (from top to bottom, with half of each cap selected but not the middle vertex from each), offset them a unit or so, then rotate it so it's at a 45 degree angle, duplicate the hell out of it, and then offset each about half a unit or so... the end result would look almost like a set of blinds... is this the kind of vent you have in mind? (making the frame all depends on the shape you want your vent to be so i won't go into too much detail besides making a curve, duplicating it and scaling the duplicate larger, making a loft between the two, and then making an extude using the larger of the curves so it's got depth)