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.