Yeah, you're right. It would be hard to make a test that was comparing apples to apples. First you have to find two nights with the same temperatures. Plus, you have to have about the same humidity. In addition, just having to add clothes to make up for the lack of a vapor barrier would add another variable right there. Who could say how much moisture the clothing you wore would absorb. But, if you could account for all the variables, it would really be a useful test. If nothing else, if you had zero added weight using the vapor barrier, and then had some added moisture without using the vapor barrier, that would tell you pretty much all you needed to know. Even if the conditions were somewhat different on the two nights of testing.
For I consider that the sufferings of this present time are not worthy to be compared with the glory that is to be revealed to us....that the creation itself also will be set free from its slavery to corruption into the freedom of the glory of the children of God. For we know that the whole creation groans and suffers the pains of childbirth together until now.