Regarding vents: For example, in the Ultra Flash I have a main and two saddle tanks. The two saddle tanks are in parallel to each other and in series with the main tank. I could not get them to work with them all in series w/o causing the UAT to collapse, even with high flow fuel system components. Each saddle tank has its own vent line which terminates outside the bottom of the fuse. There are fittings in the bottom of the fuse that the vent lines attach to. The holes through the fittings can be constraints if the holes are not large enough. Also, some folks add some filters on the vent line. These filters are another potential constraint. Sysiek is right. If you see your UAT collapsing when at full power, your fuel system is constrained.
Regarding vent line filters: I personally do not use filters. But, I do flush the vent lines into a taxi-tank every time I fill it. That is why it may not be a good idea to reuse fuel that makes it to the taxi tank, especially if your frequently flying off grass or dirt. If I was flying off grass or dirt, I would use a separate taxi tank for filling/flushing. But I have a nice asphalt 3800' runway, pit row, ramps and concrete fuel pads here. Only time I got off pavement was at Joe Nall and that made a mess of the gear and bottom of the plane (i.e. grass clippings and dirt all over the gear, flaps, hinge lines, etc.) It think it was Bob V. that once told me, "Never fly off wet grass. Better to wait till it dries." Good advice!