Vicks V3100 1.2 Gallon Humidifier leaking

Check if the cap has a gasket or o-ring and is not worn. There are two designs for this model. The older model has the fill cap and valve together. The new design the fill cap and valve are at different locations. The new design has no hole in the bottom of the unit.
You can fixed the problem using a 1-1/2 inch washer for brass tubular found in the plumbing isles in HD or L. The rubber washer is 1 or 2 mm smaller then the grooves in the fill/valve cap. You can stretch the washer and it will fit perfectly in the grooves. When putting the fill/valve cap back with the washer, it will be tighter then before. Firmly turn and you should not have any leaks and make sure unit is level. You can plug the hole because the new design does not have a hole in the bottom of the unit.

I had the same problem. Turned out to be a bad seal on the cap. I found a customer service phone number for Vicks and they are sending a new cap, at my cost of $4. If I figure out where I found the number, I'll add it to this comment.