Henbit and vinegar

Five-percent white distilled vinegar from the grocery store will kill henbit, a cool-season annual weed. But I think the vinegar needs to work in conjuction with some other factor to be effective against henbit. That factor might be warmer temps like 60 degrees plus maybe, or perhaps it's a lot of sunlight.
I've been able to kill tons of henbit in my *back*yard with a 5 percent white distilled vinegar spray, but when I sprayed henbit located in my *side* yard which gets much less sun and gets lotsa shade due to the house, the henbit would not die. I sprayed that henbit on the side of the house 2 or 3 times on 2 or 3 different days....sprayed it heavily. I really unloaded that vinegar into the henbit, but the henbit wouldn't die. It looked perfectly fine days later as if vinegar never touched it.
What happened? Does vinegar need to work in conjunction with *sunlight* to eliminate henbit? Or does the vinegar need warmer temps? I finally had to pull out the sideyard henbit to get rid of it.
Tons of henbit got wiped out in the backyard by using vinegar but the usually shaded sideyard henbit still looked strong and untouched. It laughed at vinegar.
I'm not sure what the X Factor is: sunlight or warmer temps. I tend to think it's sunlight because I believe I sprayed the side yard during warmer temperatures.
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I think distilled vinegar must need sunlight in order to kill henbit. Warmer temperatures are not the key here, but the sunlight. I now remember spraying the shaded sideyard's henbit during warmer temperatures many weeks ago, but the henbit survived fine in the shade of the house.
I sprayed some non-shaded henbit with vinegar a day or two ago, and today I've seen the henbit dying after exposure to today's sunlight. The past couple days have been cold with daytime temps between 40 and 55, but the henbit is still dying today. Sunlight seems to be necessary in order for henbit to die when exposed to vinegar. Sunlight is the X Factor.
The unsprayed henbit, by the way, still seems to be growing in these temps. Makes sense, I guess, since it's a cool-season weed.
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