CO2 IS "transparent" (colorless) at all times. The "fog" you see is frozen water vapor from the air.
CO2 is significantly more dense than air, so will fill the container from the bottom up. You'd really be better drilling a small hole in the container top, and letting the carbon dioxide "push" the air out though that alone.
Go with a larger container, and put the cups with dry ice apart from the plants, and you'll have no chilling issue, as the gas warms pretty quickly. The yeast idea won't generate CO2 quickly enough to work well, and vinegar is a herbicide, so I don't know that I'd want to risk my plants near it.
I will add that many part of pests' life cycles take place in the substrate, so while you may have effectively killed bugs on the plant, some might be in the tank still.
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