This experiment was probably doomed to failure before it began. But, I had to try.
For about $8.00 ACE Hardware sells a painters overall / suit which is supposed to stop paint and other liquids from getting on the painter (and it is very light weight - like 4 oz.).
So, I thought, maybe it could be a cheap windshirt or rain gear. (It had a full zip and little holes all over the fabric, so it had to be breathable.)
Waiting until the conditions were right, which is not easy in this environment, it finally started raining. It was supposed to snow, according to the weather report. I put on the unmodified painters' suit, not even the tags were removed to reduce weight before this test.
Front view, 42 F, no wind, average humidity for our elevation before rain or snow:
Side view, same weather:
Then, standing outside waiting to test the gear - the rain stopped.
So, I connected the garden hose to the faucet hoping to spray water in the air, emulating a rain condition. (The faucet was semi-frozen closed at first so that was quite an effort.) Finally, it sprayed water out of the connection between the hose and the faucet.
I let the faucet spray hit my right arm, then my left and backside. Immediately, it was apparent that even though this fabric breathed well, it was letting water through. So, I struggled with the faucet and got it off.
The water beaded on the surface well, but came through the little holes in the fabric pretty easily:
Conclusion: Where there is any water pressure from wind or a leaky faucet the fabric will not be an effective vapor or water or snow barrier, as evidenced by my soggy underlayer on the right arm which was most exposed to the spray; after it quit raining. (My partner suggested I get into the shower with it for a further test, I will leave that to one of stronger motivation and conviction.)
Thus, I do not recommend using a painters' pants suit for trekking gear. It also split at the seam in the crotch when I jumped back from the initial spray from the faucet.