The cool vest description says that it protects dupes from heat by decreasing insulation. However the vest adds +0.05 to dupes insulation stat increasing insulation thickness.
To test I locked a dupe in a 70C room with a space heater wearing the cool vest. They quickly gained the toasty surroundings debuff and complained about heat.
The reason I think it is broken is because when I removed the cool vest the dupes condition actually improved and toasty surroundings went away.
I believe the cool vest is actually cooking the dupe.
Steps to Reproduce
Wear a cool vest in a room with a space heater and heat up to max at around 70C. See the dupe complain about temperature. Remove the cool vest. The dupe is now better off without the vest.
This issue has not been confirmed by a developer yet.