Should You Add A Moisture Barrier To Insulation In A Bathroom Wall

Please help with this question/business about vapor barrier for my exterior wall. I have linked my original thread that I have on the Remodel Forum. Typically I would have stopped at that place with the vapor bulwark on the inside of my wall considering of my geographical location. BUT I am concerned about causing a vapor sandwich considering of the siding setup we take. Below is my final mail service on the Remodel Forum but I am not getting whatever responses. I have since read controversial articles on the topics. Such as this 1 that is at a site called Green Building Advisor that I found Friday:
http://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/vapor-barriers-are-practiced-thing-correct

Which side of the issue are your on ;)

Anyway, hither is my latest post on the tread in the Remodeling Forum and the thread is also linked beneath.

How-do-you-do, I'1000 back gear up to finalize what I am going to do with the exterior wall insulation of my s facing bathroom wall in Iowa. Fundamental Iowa is zone v on the map above that Hollysprings posted. On some other thread Worthy had a link to the building scientific discipline site and I institute some other paper written that addresses vapor barriers. I linked it below.
I will have a shower and washer/dryer in this room. I will have a 170cfm Fantech exhaust fan in the room with remote fan in the attic and 2 vents, one at the shower and the other at the Westward/D stack.

Outside WALL Clarification:

2x4 stud wall that faces the south. At that place is i window in this wall. I volition accept drywall and latex paint on the interior finished wall.

The material that is on the exterior side of the studs is 1x8 T&G boards from the xx's when the house was built. Out side of the boards is an quondam siding of pressed composite textile that is from the 70's. It looks like the original narrow wood siding was removed at that time. The concluding exterior layer was put on by Sears (over the 70's era siding) inside the last 10 years. It is a vinyl siding with a 3/4 pink extruded polystyrene insulation under the vinyl. The polystyrene material is past Owens Corning and says "Weatherb...Exterior Protecti..." (I only have a small section of textile to read).

I have been planning on using Roxul batt insulation. This requires a poly 6 mil vapor barrier sail put on the inside surface of this wall. And then the stabled areas get covered with "Constrict Tape". Of form I tin can't observe Tuck Tape in the US.

QUESTIONS:
ane) Will it be appropriate to put this vapor bulwark on the interior surface of my studs with all the sheathing materials that are on the outside studs? Or will I exist creating a vapor sandwich, causing harm to the studs?
two) Will Tyvek Tape substitue for Tuck Tape?
3) If not Tyvek Tape, what does substitute for Tuck Tape?

Hither is a link that might exist useful: HOW DO I PREP/INSULATE STUD WALL FOR Moisture PROTECTION AND INSULATION (BOTH EXTERIOR & INTERIOR)