Since you say it is a small bathroom originally you would be most likely fine to block the vent off with no real impact. If I had to guess that vent gets maybe 30-50 cfm and is probably less than a 5” duct. Your blower fan will not blow up, your system will adjust itself to the removed duct and the small amount of air will redistribute. Pressure increase would be negligible. While this is not the “proper” way it will work. Best way to block off the run is get as close to the main trunk as you can and use a cap. Second best is to at least get away from the flex duct and on the hard duct again where you can use a cap, I would attempt to avoid just zip tying that off.