Do you have a source referencing the local velocities? I'm curious to learn more detail

Also google or amazon"baking ring molds" you've got a lot of options, these look fun :)

Hose clamps for baking molds is definitely a cost effective approach, and would save you a lot of the work of cutting things. Maybe just a liner of aluminum foil or parchment paper inside the clamps would do the trick

If it can be done safely... could you secure the tube around a sacrificial, longer core (i.e. maybe pressure fit a wooden dowel) and clamp that in the saw to then cut the tube?

What's the application, and does it have to be stainless? Depending on what you're looking for possible source here
https://www.mcmaster.com/rings/material~stainless-steel/

Depends a lot on the structure/materials of your wall and you're also on the right track by noting location, proper vapor control depends on balancing the climate inside your home versus the conditions outside which obviously varies place to place (part of the reason its so hard to find one answer!). Links below are of the some of best references that I have found, hope it helps.

General rule of thumb "In areas where the climate is cold in the winter, the vapor retarder should be installed on the inner side of the wall near the warm interior space — or on the warm side in winter. In humid climates or areas where there is extensive use of air-conditioning, if a vapor retarder is required, it should be installed on the exterior side of the wall."

https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers
https://insulationinstitute.org/im-a-building-or-facility-professional/commercial/installation-guidance/managing-moisture-in-commercial-construction/vapor-retarders/