They didn't use high temp red silicone on the gap between the vent pipe and wall flashing. Not a dyi job but may be done by taking the termination off and using a silicone gun with extension on the nozzle to reach to pipe/flashing gap. It should make people mad that it would have taken 5minutes and $5 dollars at the time if installation to avoid this draft. Thats what you get when you hire a builder and installer that lacks integrity. Now your house lacks integrity.