When he says soffit it's venting into the heat system? It's not just throwing moisture into soffit area correct? Have to ask that last question because 30 years in the trades I've seen a lot. LOL!
In Washington State all vents for baths I've seen either go to roof or vent on outside wall of house.
Forget the vapor barrier. Not needed.
BTW............... Use Exterior paint in the bath. A bit more expensive paint but well worth it. Has all the properties in it to combat moisture.
Seed equals food, seed is like money in the bank. Jackie Clay