It all depends on how warm the room your viv is in, and how much additional heat you're willing to provide. My feelings on the matter are that if you have thermometers at the coolest part of the tank, the warmest part of the tank, and at or near the basking spot, you should be able to adjust your heating to get it right. Beardies have a fairly tolerant temperature range, it shouldn't be too bad to fix something up to keep everything balanced. Obviously if your viv is in a very cold room, the incoming air will be cool, but if you have plenty of heat sources it shouldn't be a problem. Beardies will move to where they are most comfortable anyways.
I personally have lower vents on my ig enclosure as well as upper, and my beardie lives with an all mesh cage top, and my temps are usually pretty stable. Outside temps can change 15-20oC overnight around here, if I know a cold snap is coming I close a couple vents or muffle them with a towel, if I know a heatwave is coming I make sure all vents are open.