These charts were all the rage a couple years ago.....and at 1.8 it says you shouldn't go to 4.0v or above or you could melt the nichrome wire or something else. I assume melting the wire means you'd pop your coil, but not sure. Oh, and I get the book after you!
http://www.ecigadvanced.com/community/wp-content/uploads/2012/07/power.jpg
Ah, the charts
Written long ago, in a much simpler time. Pre variable anything not rebuildable.
And without air flow control.
They were done to suggest reasonable ranges for the low power fixed voltage batteries, and for the limited airflow clearos, and still are a good starting point for those types of toppers. But with increased air flow more power is ok, and maybe beneficial. And we have the adjustability to keep it a tasty vape..
While it's unlikely to actually melt the kanthal, it was more to prevent burning the wicks and juice. (And yes, actually melting out the kanthal would be one example of popping a coil.)
Watts is the power used. A 1500 watt heater will put out more heat than a 500 watt one, if both have the same voltage. Easy to picture. And they're made for a specific voltage, 120 VAC in this example. If you could give them a lower voltage, they'd put out less heat. A higher voltage would put out more heat. As long as you don't change the resistance of the heater.
Or, at our 120 VAC, if you could wind a new resistor for the heater, you could change the power / heat. Not real practical, tho. But if you could vary the resistance, you could live with the change in warmth, or just keep changing the voltage until you got as warm as you want, i.e. the same heat as before.
What VW control does is automate that voltage change for you.